Christian attitudes to sexuality
The Christian view of sexuality has evolved over time. From the early church to the end of the medieval period, sex was only for procreation within marriage. Pleasure within sex was sinful and was to be suppressed or tightly controlled. Celibacy was considered preferable to marriage; marriage was better than fornication. These views were moderated to various degrees from the reformation onward. Christian views of sexuality were traditionally very negative.
Throughout history, Christianity generally teaches that sex is to only to occur within heterosexual marriage.
Contemporary Christianity now comprises a wide variety of views on sexuality ranging from very repressive to very liberal. Sexual pleasure is now not usually considered sinful. Modern Christianity is either positive or neutral about sex in some contexts (that it controls though marriage) but often traditionalist in other areas, such as non-reproductive sex, pre-marriage sex, masturbation and homosexuality. Christians often do their own thing based on local cultural norms and frequently disregard their denomination's teaching on sex.
"Be fruitful and multiply"
"The husband should give to his wife her conjugal rights, and likewise the wife to her husband."
The Bible condemns homosexual acts.
Against passion and sensuality
"But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart. And if thy right eye offend thee, pluck it out, and cast it from thee"
"abstain from sexual immorality; that each one of you know how to control his own body in holiness and honor, not in the passion of lust like the Gentiles who do not know God;"
Passion, also translated as lust, from the Greek word pathos. 
"But I say, walk by the Spirit, and you will not gratify the desires of the flesh."
"Now the works of the flesh are evident: sexual immorality, impurity, sensuality,"
Sensuality has also been translated as lasciviousness. 
The solution proposed by the New Testament is the repression of these desires:
"Put to death therefore what is earthly in you: sexual immorality, impurity, passion, evil desire, and covetousness, which is idolatry."
"And if your eye causes you to stumble, gouge it out and throw it away. It is better for you to enter life with one eye than to have two eyes and be thrown into the fire of hell."
Celibacy is superior to marriage
"I say therefore to the unmarried and widows, it is good for them if they abide even as I. But if they cannot contain, let them marry: for it is better to marry than to burn."
An partner in marriage should not unilaterally withhold sex
Within marriage, a husband and wife may not refuse sex except by consent of both parties. In other words, the Bible says mutual consent is required not to have sex. Therefore, if either partner desires sex, their partner should comply. This "commandment from God"  is at odds with the modern concept of sex always requiring consent.
"Do not deprive each other except perhaps by mutual consent and for a time, so that you may devote yourselves to prayer. Then come together again so that Satan will not tempt you because of your lack of self-control."
Early Christian writers, such as Tertullian, Ambrose, and Jerome, taught that sex necessarily had negative aspects such as lust. Christian writers often frame the sexuality morality in terms of its naturalness, which is subject to the naturalistic fallacy, and in terms of the purpose of sexuality (i.e. teleologically).
"The wise man should love his wife with cool discretion not with hot desire… Nothing is nastier than to love your own wife as if she were your mistress."
There are many accounts of self-castration in the early church. The theological arguments for self-castration, possibly based on Matthew 19:12 , seem to have been lost. The practice was general condemned by early prominent writers within the church.   Self-castration was reportedly carried out by the early Christian theologian Origen.
Augustine (354 - 430) taught that sex was only to be used for procreation. Sexual passion was to be avoided and he recommends celibacy for those who cannot limit sex to this purpose. This is in agreement with the Bible arguing that sexual passion should be repressed.
"The patriarchs possessed their wives for the work of procreation, not 'in the passion of lust like the Gentiles who do not know God.' This is so great that many today would contain themselves more easily for their whole life from all intercourse than to hold to the norm of not uniting except for offspring, if they were to be joined by marriage. [...] Who have we heard [...] saying to us that he never had intercourse with his wife except when hoping for conception?"
- — The Good of Marriage, Chapter 13 
- "I consider that nothing so casts down the manly mind from its height as the fondling of a woman, and those bodily contacts."
Augustine taught that original sin was transmitted by "concupiscence" i.e. sexual lust, which is an evil that is present in all sex. In marriage, is is a "matter of necessity" and "even marriage has occasion to feel shame". Within marriage sex, concupiscence must be restrained and solely directed to "intent on offspring".
- "Now from this concupiscence whatever comes into being by natural birth is bound by original sin "
The medieval Catholic church taught that only sex in the missionary position was (somewhat) acceptable and any other position was sinful.
- "Christian moralists, canonists, and theologians from the patristic period onward commonly maintained that only one posture was appropriate and natural for human sexual intercourse. "
- "Despite biblical silence about sexual position, church authorities from the 6th to the 16th centuries taught that, except when illness, obesity, or pregnancy dictated otherwise, sexual intercourse should be facetoface with the man on top "
Summa confessorum and penance
Dorsal sex (woman on top): three years
Lateral, seated, standing: 40 days
Coitus retro — rear entry: 40 days
Mutual masturbation: 30 days
Inter-femural sex — ejaculation between the legs: 40 days
Coitus in terga — anal sex: three years (with an adult); two years (with a boy); seven years (habitual); 10 years (with a cleric)
These penalties have not been applied for centuries. However, it does illustrate the church's obsession with control of sexuality.
In general agreement with St. Augustine, Thomas Aquinas (1225 – 1274) argues:
- "Now just as the preservation of the bodily nature of one individual is a true good, so, too, is the preservation of the nature of the human species a very great good. "
- "Now lust consists essentially in exceeding the order and mode of reason in the matter of venereal acts. Wherefore without any doubt lust is a sin. "
He also argues that sex within marriage has both good and bad aspects and the good must outweigh the bad. Aquinas says sex within marriage is somewhat like fornication.
- "the intercourse of fornication and that of marriage are of the same species as regards the species of nature. But the intercourse of fornication is wrong in itself. Therefore, in order that the marriage intercourse be not wrong, something must be added to it to make it right, and draw it to another moral species. "
He also points out the "bad" as aspects of sex: "a loss of reason incidental to the union of man and woman" and "the tribulation of the flesh which such persons have to suffer from solicitude for temporal things". 
Catholic theology has "developed" since Aquinas's time and his view is no longer mainstream in Christianity. 
However, Aquinas does not consider sex which happens to be sterile as automatically a sin.
Erasmus (1466 – 1536) reiterated the traditional view of Christianity.
- "the marriage I praise is very similar to virginity, a marriage in which one has a wife for the production of offspring, not the satisfaction of lust"
Martin Luther (1483 – 1546) was very enthusiastic about marriage and child bearing based on the biblical principle "be fruitful and multiply". He argued that priests, monks and nuns should marry. Luther was himself married. He said that, apart from eunuchs or certain rare holy celibates, "let no man presume to be without a spouse". If a husband is infertile, he also argued that the wife should seek a "secret marriage" with a close relative of the husband, so the wife my become pregnant and the husband is obligated to accept this arrangement. The principle also applies if the roles are reversed. 
However, Luther retains the earlier principles that lust in marriage is wrong:
- "Christian married folk should not permit themselves to be governed by their bodies in the passion of lust, as Paul writes to the Thessalonians [1 Thess. 4:5 ]"
- "In spite of all the good I say of married life, I will not grant so much to nature as to admit that there is no sin in it. .. no conjugal due is ever rendered without sin. The matrimonial duty is never performed without sin."
In contrast to popular perception of the puritans, they considered sex within marriage to be a duty, to be conducted "with good will and delight, willingly, readily, and cheerfully." 
"[They] do err, who hold that the secret coming together of man and wife cannot be without sin unless it be done for the procreation of children."
The Shakers are a Christian cult that teach that sexual intercourse is at the root of all sin.  At the time of writing the cult is virtually extinct, perhaps in part caused by their commitment to celibacy.
St. Alphonsus Liguori
The views of Alphonsus Liguori (1696 – 1787) have largely replaced the teaching of Augustine and Aquinas in the Catholic church. He thought that sex is "lawful" if there is "urgent need for showing signs of affection to foster mutual love".  This was influential in dispelling the Augustinian (and biblical) notion that sex was only to be for procreation and pleasure was automatically sinful.
His ideas were later expanded by John Gury and others to form the modern Catholic view.
Between Christian denominations or even within each denomination, there is little agreement on sexuality apart from marriage being the preferable (or only) context that sex many occur. In several controversial areas, some Christian leaders suggest that believers pray for guidance to find answers. Specific sex positions and acts are not usually discussed, except occasional arguments against anal sex even in marriage . As Christian dogma becomes more irrelevant to most people's lives, Christians generally ignore its teachings on sexual ethics and do what ever they want, including abortion, pre-marital sex and using contraception in non-procreational sex. There is also a diversity of views on homosexuality, depending on the denomination and individual believer. The teleological view of sex is still common:
"The biological purpose of sex is children, just as the biological purpose of eating is to repair the body."
Non-procreative sex is sin
Among the denominations of Christianity, there are a wide diversity of views on sex. Some are strongly influenced by the anti-pleasure teachings of Augustine and Aquinas. Catholic teaching is that procreation is the primary (but not only) purpose of sex. This rules out oral, anal or any other sexual act that does not have the possibility of procreation. 
- "Each and every sexual act in a marriage needs to be open to the possibility of conceiving a child. "
Catholic teaching still maintains its position that it is sinful. Pope Francis described not having children as a selfish act.  Many other Christian leaders are content to focus on fighting against fornication and do not concern themselves with non-procreative sex.
Non-procreative sex is not a sin
Most Christians do not consider non-procreative sex as sin. This view is often based on personal judgement and cultural influences rather than church teaching.
- "It is NOT a sin to have sex only for pleasure inside of a marriage. That is why God designed it that way. "
- "Sex is God's gift to married couples. "
By this logic, since God created everything, anything or at least everything pleasurable is permitted (therefore, reductio ad absurdum). Apologists claim that since God creating sex, they know that sex is special and worth protecting.
Conjugal duty is the continued sexual availability of both partners in marriage to each other.  This is to fulfil the supposed primary purpose of marriage: to produce and rear children. This view is generally unpopular due to its incompatibility with the modern concept of sex requiring consent.
Celibacy is not superior to marriage
Another Christian view rejects the notion that celibacy is superior to marriage, which was the teaching of the medieval Catholic church. This idea was promoted by Luther and Calvin and is common in many protestant denominations. In many denominations, ministers are permitted to marry.
Sex and the Fall
A small minority of Christians believe that sex was the original sin in the garden of Eden. This idea is based on a metaphorical interpretation of the Fall. Similar views were expressed by early Christian writers, such as in the apocryphal Acts of John which described sex as "an experiment of the serpent", but there were not canonised into the New Testament.  Most Christians do not accept this view and consider the fall to be caused by pride or disobedience. 
The Catholic church now teaches that there is a "unitive" purpose within marriage to strengthen the relationship, as well as the original aim of procreation. While sex is still focused on child bearing, a specific instance of sex may have an indirect connection to conception. Calendar based contraception may be used to "delay" conception. Sexual pleasure is no longer automatically condemned but is considered part of the unitive aspect of sex. This is in stark contrast to the views if Augustine and Aquinas.
- "Sexual pleasure is morally disordered when sought for itself, isolated from its procreative and unitive purposes. "
Sex without any intent to eventually produce a child is generally considered a sin. Sex with an infertile partner is permitted because there is still a remote possibility of conception. Catholic priests are required to remain unmarried and celibate.
- "To be moral, each and every sexual act must be marital and unitive and procreative. All non-marital sexual acts, all non-unitive sexual acts, and all non-procreative sexual acts are intrinsically evil and always gravely immoral. "
This would rule out anal sex, oral sex, sterilization, probably foreplay too, as well as the use of contraceptives. Many Catholic believers hold the view that these acts are acceptable, as long as both parties consent. Some Catholic theologians break with traditionalist views and propose more liberal and tolerant sexual ethics. These views have been rebuked by the Vatican. 
Although not explicitly discussed in the Bible, the vast majority of Christians condemn rape. The issue of marital rape and the role of consent is more controversial, with some Christians claiming that rape cannot occur within marriage. The denial of marital rape is sometimes based on the theology on the unity of marriage or on conjugal duty.
Some denominations promote sex education that emphasises sexual abstinence before marriage. Unfortunately, this results in more teen pregnancies and more risky behaviours than a more comprehensive sex education.
"The problem is that Christians [...] are not principally concerned about teen pregnancy and the spread of disease. That is, you are not worried about the suffering caused by sex; you are worried about sex."
- "In fact what’s getting taught is this: condoms fail constantly, sex is deeply dangerous and shameful, and “the girl you want is the girl who says ‘no’”. "
Sexually transmitted diseases
Many Christian groups, including the Catholic Church, spread misinformation about the effectiveness of contraceptives.  Some Christians are resistant to the idea of vaccinating against sexually transmitted diseases, even though there are risks of contracting them from their spouse or from sexual assault.  Some STDs are serious, with HPV being the most common in the US and is major cause of cervical cancer.
- "Many conservatives worry that giving our daughters the vaccine undermines abstinence teaching. They feel it could encourage promiscuity by disinhibiting them from having sex. Lots of teens aren't having sex, they say, so why indicate we think they will by giving them the vaccine? "
- "Reginald Finger, an Evangelical member of the CDC’s Advisory Committee on Immunization Practices, recently announced that he would consider opposing an HIV vaccine—thereby condemning millions of men and women to die unnecessarily from AIDS each year—because such a vaccine would encourage premarital sex by making it less risky. "
Christians who believe in celibacy before marriage often get married while young so they can experience sex. This can lead to hasty marriages with partners that are not well suited.
- "I bought heavily into the purity culture myth [...] I rushed into marriage at 23 mostly so I could have sex. Purity culture taught me that if I did ‘everything right’ prior to marriage, then God would bless me with a special woman that he created just for me … and we would have an amazing sex life. [...] I wanted to be a good husband and Christian, but I had this nagging feeling that I was sold a lie. "
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- ↑ Robert Crooks; Karla Baur (2010), Our Sexuality (11th ed.). Cengage Learning. p. 11. ISBN 0-495-81294-3.
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- ↑ 9.0 9.1 9.2 James A. Brundage, Let me count the ways: canonists and theologians contemplate coital positions, Journal of Medieval History, Volume 10, Issue 2, June 1984, Pages 81–93
- ↑ 10.0 10.1 
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- ↑ 12.0 12.1 
- ↑ 13.0 13.1 
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- ↑ Michael J. Mcclymond, Two Become One, Two Become Three: Pleasure and Procreation in Christian Understandings of Sex, Modern Reformation, Nov/Dec 2001
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- ↑ USCCB Catechism, p. 409
- ↑ Stephanie Kirchgaessner, Pope Francis: the choice to not have children is selfish, The Guardian, 11 February 2015
- ↑ 23.0 23.1 
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- ↑ 31.0 31.1 Harris. S. 2006. Letter To A Christian Nation p. 10-11
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- ↑ 34.0 34.1 
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- Michael J. Mcclymond, Two Become One, Two Become Three: Pleasure and Procreation in Christian Understandings of Sex, Modern Reformation, Nov/Dec 2001
- James A. Brundage, Let me count the ways: canonists and theologians contemplate coital positions, Journal of Medieval History, Volume 10, Issue 2, June 1984, Pages 81–93
- Neil Carter, Sex and Christianity Make Strange Bedfellows, November 29, 2013