Rape is committing a sexual act without consent. It usually involves penetration upon another person, through violence, threat of injury, or other duress, or in cases where the victim is unable to decline due to physical or mental incapacity, possibly due to the effects of drugs or alcohol. (Statutory rape involves sex with a minor, whether or not the act was coercive.) Rape is considered one of the most serious sex crimes in almost all jurisdictions.
Both the Bible and Qur'an do not address rape directly but consider fornication and adultery as immoral. Since the latter are not criminal acts in many jurisdictions, rape is a distinct (and largely secular) concept. Neither holy books address the issue of marital rape but rather discourage celibacy in marriage. Their basic presumption is that people in marriage have a right to sex, rather than a right to not be raped. The issue of consent is generally not discussed. In the rare instances in which it is discussed they imply that an individual cannot unilaterally withhold sex. Consent is a core principle of social and legal standards for sex in many countries. It is quite telling that various holy books neglect to discuss it.
Because of the extremely black-and-white ethical nature of most cases of rape, it is frequently brought up in discussions about morality.
The Old Testament of the Bible contains many stories and laws which appear to condone rape, particularly in war time. Frequently, the Israelites are said to have visited neighboring tribes and committed wholesale slaughter, leaving alive only young virgin women to take unwillingly as wives.
This practice is explicitly endorsed in Deuteronomy 20:10-14 :
- "When you march up to attack a city, make its people an offer of peace. If they accept and open their gates, all the people in it shall be subject to forced labor and shall work for you. If they refuse to make peace and they engage you in battle, lay siege to that city. When the LORD your God delivers it into your hand, put to the sword all the men in it. As for the women, the children, the livestock and everything else in the city, you may take these as plunder for yourselves. And you may use the plunder the LORD your God gives you from your enemies."
Numerous examples can be found where this law is followed. Numbers 31:15-18 says:
- "'Have you allowed all the women to live?' he asked them. 'They were the ones who followed Balaam's advice and were the means of turning the Israelites away from the LORD in what happened at Peor, so that a plague struck the LORD's people. Now kill all the boys. And kill every woman who has slept with a man, but save for yourselves every girl who has never slept with a man.'"
Judges 21:10-12 similarly says:
- "So the assembly sent twelve thousand fighting men with instructions to go to Jabesh Gilead and put to the sword those living there, including the women and children. 'This is what you are to do,' they said. 'Kill every male and every woman who is not a virgin.' They found among the people living in Jabesh Gilead four hundred young women who had never slept with a man, and they took them to the camp at Shiloh in Canaan."
What did the Israelites intend to do with these women? Lest there be any doubt, we can go back a few verses to Judges 21:7 , where they asked
- "How can we provide wives for those who are left, since we have taken an oath by the LORD not to give them any of our daughters in marriage?"
The entire purpose of the attack was to obtain women as wives. It's hard to argue that the women would have been willing spouses after seeing their village destroyed.
Apart from rape as spoils of war, sex outside of marriage is forbidden. While fornication usually carries the death penalty, circumstances that imply rape result in no direct punishment of the rape victim Deuteronomy 22:28-29 . However, the rape victim is not betrothed, the rape perpetrator and victim must be married!
- "If a man find a damsel that is a virgin, which is not betrothed, and lay hold on her, and lie with her, and they be found; Then the man that lay with her shall give unto the damsel's father fifty shekels of silver, and she shall be his wife; because he hath humbled her, he may not put her away all his days."
This would be extremely traumatic and is not considered as a reasonable solution today. This is an obsolete law dating from a time in which marriage was largely an economic/reproductive arrangement. It is also an example of the selective use of Old Testament law. The Old Testament does not seem to address a woman raping a man.
Alleged rape of Mary by God
Some writers have accused God of raping Mary. However, Mary appears to know of God's plan before it occurs and agrees to it.  Luke 1:30-38 Since most skeptics do not accept that a virgin birth took place, the accusation of rape by God is not a relevant issue. Based on the same events, some writers have accused God of committing adultery.
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. This raises the possibility of marital rape.
"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."
The official teaching of all mainstream denominations and the view held by the vast majority of Christians is that rape is immoral.
"Rape is the forcible violation of the sexual intimacy of another person. It does injury to justice and charity. Rape deeply wounds the respect, freedom, and physical and moral integrity to which every person has a right. It causes grave damage that can mark the victim for life. It is always an intrinsically evil act."
- — Catechism of the Catholic Church 
However, many denominations are slow to the acceptance that consent is the principal factor in deciding if sex is rape. Consent is not mentioned in the Catholic Catechism with respect to rape. There are sporadic debates as to if the Catholic Church should officially recognise and ban marital rape.
The concept of rape is not explicitly expressed in the Qur'an. Therefore, it is not explicitly forbidden by the Qu'ran. The Qur'an does instead ban fornication and adultery, which are not the same thing.  There are passages in the Qur'an in which rape is sanctioned in some circumstances.
"O 'Uthman! Allah did not send me [Muhammad] for monasticism, rather He sent me with a simple and straight [shari'ah]. I fast, pray and also have intimate relations with my wife. So whosoever likes my tradition, then he should follow it; and marriage is one of my traditions."
- — Wasaelush Shia, Vol. 14, p. 10 
"and your wife has a right over you."
According to the Qu'ran, slaves are helpless before their masters Surah 16:77 , can be bought and sold, and masters can have sex with female slaves whenever they want Surah 4:3 , which would be considered as rape in many jurisdictions.  
Sharia law generally prosecutes rapes under Zināʾ laws, which cover fornication and adultery regulations (rape is not banned separately). They usually require 4 witnesses or a confession for a prosecution to succeed. Since this is generally not available, prosecutions rarely succeed. Some courts reject forensic evidence. In many cases, a failed prosecution will result in a prosecution of the accuser for "adultery". 
- "When and if this happens [prosecution of rape victim], it does not represent Islam but represents cultural understanding and man made laws, not God's commands in the Quran. "
- "One is required to produce four witnesses when making an allegation of adultery against another person "
Regarding marital rape, people are to treat each other with respect. However, celibacy within marriage is discouraged. Some Muslim leaders go as far as to deny that marital rape is a meaningful concept.
"Clearly there cannot be any rape within the marriage. Maybe aggression, maybe indecent activity... Because when they got married, the understanding was that sexual intercourse was part of the marriage, so there cannot be anything against sex in marriage. Of course, if it happened without her desire, that is no good, that is not desirable. [...] In Islamic sharia, rape is adultery by force. So long as the woman is his wife, it cannot be termed as rape. It is reprehensible, but we do not call it rape."
- "As for "marital rape", Islam teaches both husband and wife the understanding of having to minimize the times when they are reluctant to respond to their spouses' sexual demand. Unless there are really serious reasons concerning, for example, health or mental difficulties, they should not refuse such a demand. [...] Therefore, Islam urges both parties to be always welcoming to meet the other party's sexual demands and needs. Both parties are also obliged to make sure that this affair has ended with full satisfaction for ones' partner. "
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- ↑ Catechism of the Catholic Church
- ↑ 4.0 4.1 
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- ↑ Why Sharia is an Evil Abomination, October 30, 2007
- ↑ Slavery in Islam
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