A few years ago, I wrote the following summation of this issue:
Here's my paraphrase (i.e. humorous distortion) of the five most common reasons given [against same sex marriage]. I fully expect anyone opposed to same-sex marriage to recognize their own argument here and be offended by my oversimplification of it.
1) Heterosexuals won't take marriage seriously if they have to share it with gay people.
2) Letting gay people enjoy monogamous relationships might make us stop hating them.
3) If we let gay people marry, we'll have to let rocks marry donkeys. And while individual rock-donkey couples might make decent parents, we certainly can't put them on equal footing for things like adoption rights when there are thousands of heterosexual couples out there right now waiting for a cute blonde baby to show up on the adoption list, and if the rock-donkey couples take all the cute babies, the heterosexual couples are going to have to start paying Asian agencies even more than they already are to avoid adopting ugly middle-schoolers.
4) My church doesn't marry gay people. My church doesn't marry Catholics, either. So I'm also opposed to Catholic marriage.
5) Marriage is defined as being "a man and a woman." Somewhere. In some dictionary. And I can't afford a new dictionary, so I hate it when words change.
Note that I'm not attempting to explain why we SHOULD hate gay people (or, if you must, the "sin" of gay behavior, not the sinner); that's just a premise on which argument #2 is founded. But there's arguments for that, too, both secular and religious:
1) God says so, and we don't want to piss off God.
2) If we stop hating gay people, we'll all turn gay. And then we'll go extinct.
3) Gender roles are essential to our society, which we define as a society in which gender roles are essential.
MY solution? Get rid of marriage entirely as a legal institution, replaced by civil unions for all. If people want to get married in a church, let 'em -- and let the churches apply whatever standards they want to that completely ceremonial, um, ceremony. But then make them sign a little paper that makes their "marriage" also a "civil union."
But, unfortunately, if you look at the REAL problems (as stated above *grin*), you realize why this isn't much of a "compromise" -- even if it fully solves the problem as usually phrased by SSM opponents.