The only reason I can think of to prohibit gay marriage is because of religious reasons. Virtually nowhere in the bible does it mention outright that homoloveuality is wrong. However, God ordained marriage as a pairing between a male and female, and if you believe God is perfect in his decisions, then your belief should be that marriage should only be between a male and female. Knowing that, in the US, the Constitution says "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof." Therefore, just because gay marriage goes against the beliefs of a religion doesn't mean that that religion has a right to create a law to coincide with its beliefs. And because gay marriage doesn't prevent the the free exercise of that religion means that there is no basis on which that religion can stand to oppose gay marriage legally.
This is coming from a Christian who thinks gay marriage is wrong.
Also, out of all the posts I've read in this thread, it appears that Solid and I are the only ones with actual factual evidence.
Lev. 18:22, "You shall not lie with a male as one lies with a female; it is an abomination."
Lev. 20:13, "If there is a man who lies with a male as those who lie with a woman, both of them have committed a detestable act; they shall surely be put to death. Their bloodguiltness is upon them"
1 Cor. 6:9-10, "Or do you not know that the unrighteous shall not inherit the kingdom of God? Do not be deceived; neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor homoloveuals, 10nor thieves, nor the covetous, nor drunkards, nor revilers, nor swindlers, shall inherit the kingdom of God."
Rom. 1:26-28, "For this reason God gave them over to degrading passions; for their women exchanged the natural function for that which is unnatural, 27and in the same way also the men abandoned the natural function of the woman and burned in their desire toward one another, men with men committing indecent acts and receiving in their own persons the due penalty of their error. 28And just as they did not see fit to acknowledge God any longer, God gave them over to a depraved mind, to do those things which are not proper."