There have been a number of studies hinting that sexual preference is genetic. It hasn't been proven, but I think it's logical. Why would someone choose to be gay in this society? And when did any of us choose to like girls? One day Mary Jane's cooties become Mary Jane's boobies -- there's nothing we really do to choose that.
The problem I have with Matt's arguments is that, as a straight person, he's entitled to a distinct economic benefit if he gets married, a right a gay person cannot have unless he or she enters a sham marriage. If you believe, as I do, that homosexuality is an inborn trait, it's essentially like discriminating against someone because of their race, gender, or biological disability.
With Leebo's comments, the issue is that any two people of the opposite sex can get married for almost any reason and have it recognized. You have a good night at the blackjack table, you meet that special somone, and a minister in an Elvis costume has the legal authority from the state of Nevada to marry you. The ONLY requirement is that one person has an outie and one has an innie.
In my mind, you have to either let gay couples marry or remove the state-sponsored benefits to married couples (as Fatpicther suggested). I'd much rather see the former happen because I believe marriage should be encouraged.
I would like to see the legal side of it not labeled marriage at all. Make it a civil union. I don't think it would be abused any more than marriage is now, because the legal ramifications of divorce would still be in effect, and you'd have to take legal steps to dissolve the union. This would change nothing for people marrying in the church. You'd have your ceremony, then get your civil union certificate from the state. Churches would still be free to set parameters for their private ceremonies.
Consent would still be required for these unions, so this would preclude people from "marrying" their pets, minors under the age of consent, or their XBox 360s

You can make a biological argument against close relatives marrying, too. The one issue that would be tough would be polygamy. I could see that being challenged. But then again, if someone volunteers to have another spouse, you could have them committed
