Well, I'll explain briefly.
I believe it's possible to love a sibling like no other. I believe it's possible to love the same sex like no other. I believe it's possible to love two people equally regardless of sex. So, if one believes these things to be true, one really should have some logical objection to the union of said people or they should just STFU. And here you go:
Sibling: They've known one another their entire lives. No one but them knows their exact history. It's troubling to me, even in sterile situations, because there is the possiblity that one holds some sort of sway over the other. "Hey, sis, get me my Legos", "Hey, sis, get me a soda", "Hey, sis, suck my dick", "Hey, sis, we're getting married." I'd rather deny a million "legit" sibling marriages than have one that began forming when both were still poopin' their pants.
Same sex: No real objection to man/man or woman/woman marriage. But the overwhelming reason for it is "They're in love. They want to be married. Let them be happy." None of which is a problem but it isn't applicable to just this particular marriage. So I believe it cracks a door to the next one.
Polygamy: The marriage of multiple people. As I mentioned above, I think it's possible for consenting adults to love more than one person. To love them so much they want to marry them. So, if we apply "They're in love. They want to be married. Let them be happy", we have polygamy. Much like everything in our lives, we want more. So it would quickly evolve from one man/two women or two men/one woman. We'd have multiple people. And, if you buy into "They're in love. They want to be married. Let them be happy", you can't restrict the number. Now you've got hundreds of people legally married to one another. I see that as problematic. And, because of this, I need more than "They're in love. They want to be married. Let them be happy" to get behind gay marriage. Having no opinion seems to be the appropriate position for me. I'm not going to object to it but I'm not marching on DC in support either.