I don't really see where the arguement against gay marriage is. Allow me to elaborate.
Homosexual relationships have been around more literally thousands of years. Many ancient Greeks (who invented democracy, ironically enough) were involved in homosexual relationships, and it was accepted back then. This was all, of course, long before the birth of Christ or Christianity, which brings me to my next point.
Many of those against gay marriage site the Bible as the reason for their stance on the issue. Those that do this, obviously know very little about their own religion, sadly. Yes, there are specific passages that condem homosexuality, I know there's one in particular somewhere in Leviticus. However, the two main points the Bible makes are:
1. Love God above all else
2. Treat others the way you'd want to be treated.
I'd like to put emphasis on that second one, and, well, you can fill in the blanks from here.
Here are some interesting pieces of scripture:
Leviticus 19:18
" 'Do not seek revenge or bear a grudge against one of your people, but love your neighbor as yourself. I am the LORD ."
Zechariah 8:17
"do not plot evil against your neighbor, and do not love to swear falsely. 'I hate all this,' declares the LORD ."
Mark 12:31
"The second is this: 'Love your neighbor as yourself.'[ 12:31 Lev. 19:18] There is no commandment greater than these."
Mark 12:33
"To love him with all your heart, with all your understanding and with all your strength, and to love your neighbor as yourself is more important than all burnt offerings and sacrifices."
Luke 10:27
"He answered: " 'Love the Lord your God with all your heart and with all your soul and with all your strength and with all your mind'[ 10:27 Deut. 6:5] ; and, 'Love your neighbor as yourself.'
Romans 13:9
"The commandments, "Do not commit adultery," "Do not murder," "Do not steal," "Do not covet," and whatever other commandment there may be, are summed up in this one rule: "Love your neighbor as yourself."
Romans 13:10
Love does no harm to its neighbor. Therefore love is the fulfillment of the law.
Galatians 5:14
The entire law is summed up in a single command: "Love your neighbor as yourself."
James 2:8
If you really keep the royal law found in Scripture, "Love your neighbor as yourself," you are doing right.
Luke 6:42
How can you say to your brother, 'Brother, let me take the speck out of your eye,' when you yourself fail to see the plank in your own eye? You hypocrite, first take the plank out of your eye, and then you will see clearly to remove the speck from your brother's eye.
And if you think about the whole thing logically, two guys or two girls getting married does not effect you or me in any way, shape or form. There would be no more gays running around if marriage was legalised than there are now, despite what some fools seem to think. And really, amending the Constitution or making any laws based offof one religion defeats the purpose of having "Freedom of Religion" in the US Constitution.
So if you're against gay marriage, then I urge you, please... please, do not marry a person of your same gender.