Okay, since nobody cares for religious arguments, I'll give you a few of the non-religious.
Firstly, marriage is an institution between a man and a woman that goes back thousands of years. It's a very old tradition that in modern times seams to have lost a lot of meaning for people. The reason a lot of marriages break up is because these vows weren't taken seriously by at least one of the partners. 'In sickness and in health', 'for richer or poorer', 'for better or worse', these are not just frilly words - they actually do mean, stick together even if the shit hits the fan, whether that's a year later or ten years later.
My point is, there are three plausible reasons why gay people would want to get married:
a) Because they earnestly love each other, and want to commit themselves in a LIFELONG bond as what should be expected with straight couples
b) They want equality for equality's sake
c) They want the lavish wedding day
Everyone claims they fall under category A, and while that's true for some gay couples who do earnestly love each other, it's hard to justify undoing a thousands-old tradition to accommodate such inward, selfish motives in B and C.
Furthermore, just because there may not seem to be many plausible reasons against it, doesn't mean there's a smattering of reasons for it either. I'm not against the legal rights for same sex couples, and I believe civil unions should have the same legal standings and benefits as a marriage. Aside from that, what's so crucial for same sex couples to obtain a marriage? So they can sleep easier knowing that they're not slighted in any way?
One of the more laughable arguments not just for same sex marriage, but for just about anything, is the sentiment that all previous cultures were stupid and closed minded, and we are superior with our open-minded view. In what way is our culture more advanced really? Since when does a generation that sits down to watch the Jersey Shore and laugh at a dog who walks on two legs on America's Got Talent have the right to call itself culturally enlightened? If the ancient Egyptians, Romans and other cultures in Europe had access to the technology of our day, if more of their people were schooled and they had better medical services, believe me they wouldn't be wasting away like our culture is today. If you took the workforce that built the great pyramids and gave them a tour of our culture they'd say that we're a fat, lazy, hedonistic people overall. Just because a culture may seem primitive to you, doesn't mean they were necessarily dumber. Some of you are blind to your own blindness.
The arguments on both sides, really, are quite thin. It's symbolic of the facebook era we live in, where we can have no real firm opinions, but merely 'like' an issue. Don't give a damn? Doesn't mean you can't 'like' it. That's not the hallmark of decisive action by the populous.