The New Testament
However, in at least one passage in the NT, marriage is defined as monogamous. In Mark 10:2-12), Jesus is quoted as saying:
10:2 And the Pharisees came to him, and asked him, Is it lawful for a man to put away his wife? tempting him.
10:3 And he answered and said unto them, What did Moses command you?
10:4 And they said, Moses suffered to write a bill of divorcement, and to put her away.
10:5 And Jesus answered and said unto them, For the hardness of your heart he wrote you this precept.
10:6 But from the beginning of the creation God made them male and female.
10:7 For this cause shall a man leave his father and mother, and cleave to his wife;
10:8 And they twain shall be one flesh: so then they are no more twain, but one flesh.
10:9 What therefore God hath joined together, let not man put asunder.
10:10 And in the house his disciples asked him again of the same matter.
10:11 And he saith unto them, Whosoever shall put away his wife, and marry another, committeth adultery against her.
10:12 And if a woman shall put away her husband, and be married to another, she committeth adultery.
One reader commented that this passage proves that Jesus "hated [gays]". but I'm not sure how he came away with that conclusion. If you take this at face value, it says that remarriage after divorce is equivalent to adultery. The passage 10:6-9 is just a restatement of the passage from Genesis, leading up to the conclusion 'let no man put asunder'. In 10:10-12, Jesus explains the concept again, just in case we missed the point the first time around. As usual, the language attributed to Jesus is very specific and transparent.
Also of interest is 1 Timothy 4:1:
4:1 Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils;
4:2 Speaking lies in hypocrisy; having their conscience seared with a hot iron;
4:3 Forbidding to marry, and commanding to abstain from meats, which God hath created to be received with thanksgiving of them which believe and know the truth
4:4 For every creature of God is good, and nothing to be refused, if it be received with thanksgiving:
4:5 For it is sanctified by the word of God and prayer.
Here Paul disapproves of the prohibition of marriage, as well as the practise of vegetarianism. This is probably a reference to a Gnostic group, some of whom were vegetarians. A few Gnostics and early Christians were opposed to marriage in any form (including monogamous, heterosexual marriage). Marriage was considered a grave sin by some of the early Church fathers, and the only way into the kingdom of heaven to be the lifelong mortification of the flesh. This passage by Paul from the Epistles weighs in against this particular concept. On the other hand, some Gnostics and early Christians practiced group marriage, taking 'holding all things in common' to the extreme.
One wonders about Paul's condemnation of vegetarians. Will some future US President float a constitutional ammendment mandating that vegetarians eat meat? Will conservative Christians organize abusive demonstrations at health food stores? Does God hate tofu?
The sanctioned form of marriage in Judaism and Christianity has continued to evolve over the centuries. Policies on divorce have varied widely. There was a liturgy for same sex unions in one branch of the Eastern Orthodox church. During the Middle Ages and well into the renaissance, the vast majority of European marriages were 'common-law,' and had no religious sanction: church weddings were far too expensive for most people. Mormons originally practised polygamy, although they ceased that as a condition for Utah statehood. Today, same sex unions are consecrated in some liberal Jewish and Christian denominations.
In general, society has changed the definition of marriage widely, and religion has followed by sanctioning it.
Some interpret the passages above to imply condemnation of gay marriage, or to justify their prejudices against LGBT people. The reader is encouraged to look at the entire context and make up their own mind.