Marriage is defined as:
- The legal union of a man and woman as husband and wife.
- The state of being married; wedlock.
- A common-law marriage.
- A union between two persons having the customary but usually not the legal force of marriage: a same-sex marriage.
The last definition is highly controversial, as gay marriage is a hot topic in US politics. Interfaith marriage is a sensitive issue in many communities.
Many religions do not recognise the concept of marital rape.
Interfaith marriage is a taboo in many societies and religions, particularly for a women marrying outside the faith.
Sharia law places restrictions on interfaith marriage. Muslim men are generally disallowed to marry non-Muslim women (Surah 2:221 ) but an exception is made for the "people of the book" i.e. Jewish or Christian women. Any children must be raised as Muslims. 
Most Muslims say they would not be comfortable if their children were to marry a Christian. 
While polygamy is allowed in Islam, it is a minority practice. It is banned or regulated in some areas and generally not recommended by mainstream Islam. Even though it is illegal, there are an estimated 20,000 polygamous Islamic marriages in the UK. 
To use a partner's own church as marriage venue, there are often regulations on inter-religion and inter-denominational marriage that must be satisfied. Some support groups exist that attempt to facilitate these marriages, such as Interchurch Families.
An inter-denominational marriage that involve a Catholic requires the permission of a priest for the marriage to be recognized by the Catholic Church. Permission usually requires an undertaking from the Catholic partner to raise any children within the Catholic Church and the non-Catholic partner to be aware of this commitment. If the marriage is to take place at a non-Catholic church, separate permission is also needed.  An unrecognized marriage can be later recognized or "convalidated" by the Catholic Church by following certain procedures. The practical implications of having a non-recognized marriage are minimal but it can be spiritually significant to those involved.
Many churches do not marry divorcees or have special procedures in place for this circumstance. Except for a few minor sects, Christian marriages are monogamous.
Marriage is a religious institution
Christians claim that marriage is strictly a religious tradition, and that government should not be involved in marriage at all.
The early Christian Church considered celibacy to be superior to marriage and had no specific marriage rite.
- "The early church had no specific rite for marriage. This was left up to the secular authorities of the Roman Empire, since marriage is a legal concern for the legitimacy of heirs. When the Empire became Christian under Constantine, Christian emperors continued the imperial control of marriage, as the Code of Justinian makes clear. When the Empire faltered in the West, church courts took up the role of legal adjudicator of valid marriages. "
In fact, marriage only became a church ritual around the 11-12th century. People have been getting "married" for most of mankind's history and has always also involved family, secular affairs, inheritance, jealousy, and questions of paternity.
- "Before the eleventh century there was no such thing as a Christian wedding ceremony in the Latin church, and throughout the Middle Ages there was no single church ritual for solemnizing marriage between Christians. "
In the 16th century, the Catholic church passed the Tametsi decree, which declared among other things that no marriage was legitimate unless recognized by the church. Thus, the church declared themselves effectively in charge of families by fiat.
The claims many theists have that marriage is a religious matter and that government should stay out of it could be correct. However, governments are often involved--as is the case in the United States--in what has been called marriage due to medical decisions, tax filing among many other things afforded to couples who are married. Since we live in a society that uses marriage in these ways, rather than just two people who are "married" in a religious ritual, anyone should be able to get married legally in order to be given the same rights, same-sex or not.
Same sex marriage
Same sex marriage is the union of two people of the same gender and are typically homosexual in nature. Many religions refuse to conduct same sex marriage ceremonies or to recognize the validity of such marriages. The arguments for and against same sex marriage are largely similar to apologetics relating to homosexuality.
Contrary to the purpose of marriage
- "[Same sex marriage] also denies the specific primary purpose of marriage: the perpetuation of the human race and the raising of children. "
This is a teleological argument based on the suppose purpose of marriage. However, the assumption that marriage has a particular "purpose" has not been established. Marriage is a human invented concept and only has meaning that we bestow on it.
- "[Heterosexual marriage] naturally tends to create families. On the contrary, same-sex “marriage” is intrinsically sterile. "
This variation of the argument commits the naturalistic fallacy. Just because something occurs in nature, does not in itself make it desirable.
Apologists claim that same sex marriage violates natural law.
- "Marriage is not just any relationship between human beings. It is a relationship rooted in human nature and thus governed by natural law. "
There is no grounds to believe any "natural law" exists, except in the human mind. There is also little agreement as to what natural law entails. It is also difficult to justify any such natural law without committing the naturalistic fallacy. 
- "It is in the child’s best interests that he be raised under the influence of his natural father and mother. This rule is confirmed by the evident difficulties faced by the many children who are orphans or are raised by a single parent, a relative, or a foster parent. "
Research shows that having two parents, either same sex or heterosexual, is about the same for children.
- "On the basis of a remarkably consistent body of research on lesbian and gay parents and their children, the American Psychological Association and other health, professional, and scientific organizations have concluded that there is no scientific evidence that parenting effectiveness is related to parental sexual orientation "
Validates and promotes deviant lifestyles
That assumes homosexuality is a choice, which it is not. Since it is not a choice, it does not matter if this lifestyle is validated or promoted since it will not significantly influence people's behaviour. It's like saying an owl festival would encourage people to become owls, which is absurd.
Devalue traditional marriage
Apologists argue that same sex marriage devalues traditional marriage. 
This is a vague statement: what does the apologist mean by "devalue"? The existing heterosexual marriages can continue in exactly the same way as before. No harm, no foul.
Weaken public morality
Apologists argue that same sex marriage weakens public morality. 
This argument assumes that homosexuality or same sex marriage is immoral, which is not generally accepted. The concept of "public morality" and how it may be "weakened" is vague and arguably meaningless.
Subverts state support for marriage
- "One of the main reasons why the State bestows numerous benefits on marriage is that by its very nature and design, marriage provides the normal conditions for a stable, affectionate, and moral atmosphere that is beneficial to the upbringing of children—all fruit of the mutual affection of the parents. "
This argument is an argument from authority that the purpose of marriage is to raise children. However, the state is not a generally recognised authority on this matter.
The state supports childless marriages. Based on this argument, these couples are exploiting the state by taking benefits and not spending them on children. This is absurd.
If it were necessary to only support child rearing marriages, the state could change regulations so only these families receive state support.
Violates freedom of conscience
- "In every situation where marriage affects society, the State will expect Christians and all people of good will to betray their consciences by condoning, through silence or act, an attack on the natural order and Christian morality. "
Freedom to persecute others is not a generally recognised freedom. Freedom of conscience is not absolute: for instance, it does not extend to owning slaves. Society and governments can limit what is acceptable behaviour and it does not need to defer to freedom of conscience.
Thin end of the wedge
- "same-sex “marriage” serves to validate not only such unions but the whole homosexual lifestyle in all its bisexual and transgender variants. "
- "If homosexual “marriage” is universally accepted as the present step in sexual “freedom,” what logical arguments can be used to stop the next steps of incest, pedophilia, bestiality, and other forms of unnatural behavior? "
This is a thin end of the wedge argument, which is of questionably validity. Event A (same sex marriage) is argued to be undesirable because it would lead to Event B (an undesirable outcome), or at least make it more likely. The problem is that the inevitability or increased likelihood of Event B occurring because of Event A has not been established. Same sex marriage does not automatically make incest, pedophilia or bestiality occur. We can allow same sex marriage but to continue to forbid incest, pedophilia and bestiality.
It offends God
Apologists argue against same sex marriages because it supposedly offends God. 
This assumes God exists, God has a morality in mind, humans could know what God has in mind and humans should obey God. None of this has been established reliably.
- "Calling something marriage does not make it marriage. "
The meaning of words has changed over time. If a word is used in a particular way or context, that is the meaning of the word. As Wittgenstein argued, "meaning is use". Calling something marriage does, in a sense, make it marriage as long as we have a shared understanding of the usage. You don't necessarily need to agree with the usage to understand it.