I am madly, passionately, desperately in love1.
How can anyone object to that? What could possibly be wrong with it? Well, according to almost everyone, nothing. I love her and she loves me and as long as we're happy, no one else has any complaints to raise.
Ah, but if she were a man or I were a woman, suddenly everything would be different.
Well, no, It wouldn't. That's why I don't understand how anyone who's been in love can be a homophobe, or can oppose same-sex marriage specifically2. There's really no difference between our relationship and a same-sex relationship that anyone not involved has to worry about.
And yet, in 12 states, with as many as seven more following suit this year, there is a fundamental difference: people in same-sex relationships will not be allowed to marry. In fact, they won't even be allowed to marry in Massachusetts, where same-sex marriage is recognized, because that state won't marry residents of states that don't recognize same-sex unions.
It's one thing to outlaw the act itself, the one thing that distinguishes all homosexual relationships from all heterosexual relationships3. But we no longer do that (and rightly not). What we do is arbitrarily tell certain people "even though you feel the same way about each other that those people do over there, they can get married and you can't."
UPDATE: This guy, for example, clearly has no soul.
1Well, it's been a year, most of the desperation is gone, but the rest still holds.
2As opposed to being against any formal recognition of people pairing off (or tripling off, or quadrupling off, or whatever).
3To wit, sexual activity in which all participants are the same sex