It's marriage. It's a tradition, not something which you can derive rationally. Let's hear your rational explanation of the role of marriage. Be sure to explain why it follows logically that it should only apply to groups of two or more.
If you're honest with yourself, you recognize all of this language about rationality as rhetoric in a culture war in which you're attempting to shift cultural norms.
Marriage is not solely a tradition, or there wouldn't be a problem here, as anybody could claim themselves to be married and the world would go on. Instead, it's tied heavily into the legal system, as well as into policies of organisations across the world.
When people say that gay people are not to marry their partners, that is discrimination under the legal system, entirely ignoring any cultural aspects.
Religious marriage may be a tradition. Civil marriage, though, isn't, and that's what's at stake here. For the full reasoning, See page 25 of Friday's federal court decision on Michigan's anti-gay-marriage amendment: http://www.freep.com/assets/freep/pdf/C4220110321.PDF
In the United States, the tradition of marriage excluded interracial marriage until rather recently as well. Just because something is a tradition doesn't mean it morally right.
If you're honest with yourself, you recognize all of this language about rationality as rhetoric in a culture war in which you're attempting to shift cultural norms.
I'm for gay marriage.