Other religions aren't asking us to condone their beliefs. We "allow" other religions, just like we "allow" people to be gay, but the gays want us to recognize that relationship as a marriage. Other religions aren't asking us to preach their form of the gospel. There may be other reasons, but right now that's just what popped into my head.
That word "allow" there is really, really bad. As if you could snap your fingers and disallow it. Your attitude goes against the very concepts that the United States were founded on.
People aren't allowed to practice their religion because the Christians allow them to, they are allowed to because we believe in the fundamental right for them to be able to. The founding fathers understood that if you promote one religion above the others or don't provide the freedom to practice freely, that their ability to be "allowed" to practice their religion the way they saw fit could be taken away.
You do realize that the same faith and belief that you are correct in your religion is shared by Muslims for their religion, or Jews for their religion? We can't prove any one to be the "correct" one, yet you use the word "allow" as if they are less than you somehow. What if I said I "allow" you to be a Christian? How does that sound?
Homosexuals that want the right to marry, and according to the Supreme Court of the United States in Loving vs Virginia is it a right. How can we deny them that right? They are not asking you to endorse their marriage, or bless it, or even participate in it. They are only asking that they are afforded the same rights as everybody else. I truly do not understand where you are coming from.
What if someone decided you couldn't get married? How would you feel about that? Don't jump up on your soapbox about how you'd want to marry a man, and that's how it's always been, and hasn't the past just been find and dandy because it hasn't been fine and dandy.
In 1967, a black man and a white woman (or a black woman and a white man), could not be married in sixteen states. The SAME arguments against gay marriage were used then. People considered it to be an affront to God or against the "natural order of things". Do you consider that marriage to be wrong for some reason? I would guess not since our culture has moved on since then.
Why are you so caught up in this anyway? What is the threat? What is the problem? Do you really think the US will fall into the sea if we grant people marriage rights?