Politically, I am totally pro-gay marriage. It is absolutely ridiculous that gay couples are denied the same rights as their straight counterparts. If religious denominations do not wish to recognize or bless gay unions, that is their perogative. But marriage isn't a religious institution, so their opinion should be of no consequence. Though when people think "marriage" they think church and white and all of that, marriage is really about that certificate you're issued down at town hall. You can get a religious blessing, but a relationship is only certified as marriage with a government license. All the government has to do is issue the same license to whatever couple comes in, no matter the genders of the applicants.
Personally, however, I'm deeply ambivalent about gay marriage. I don't have anything against marriage, as some do, but I'm not sure it's right for me. And it's not quite true that advocating gay marriage is simply advocating for the right for gay marriage. Straight people are expected to get married at some point. If they don't, they're treated as having something wrong with them. A lot of people get married simply because they are "supposed" to get married. There's a lot of subtle, and not-so-subtle, social pressure. I'm not really certain I want the same pressure to be brought to bear on me.