I'm against gay marriage. I'm also a lesbian.
Basically, I think getting married is a pretty silly thing to do, whether you're gay or straight. Historically speaking, marriage was a contract designed to pass ownership of a woman on from her father to her new husband (hence, her name changes from her father's to her husband's). But, if that isn't enough to put you off, there are plenty of other reasons not to get married.
For example, in most Western countries, social security rights are determined according to a 'breadwinner' system, where one member of the family is considered to be the 'head' and the others are awarded their rights according to their relationship to him. I say 'him' because, historically, only men have been considered capable of being breadwinners, this despite the fact that plenty of women are currently and always have been the sole or primary providers for their families. The extent to which this model applies varies somewhat according to where you live (e.g., it's pretty strongly applied in Britain and pretty weakly applied in some Scandinavian countries; here in France it's somewhere in between, but even in non-European countries with different histories of social security like Australia and the US you can usually find some version of the same story), but it's always there and it creates a whole heap of problems.
Marriage encourages people to be dependent on another, economically speaking: in short, for women to be dependent on a man. But what about divorce? Divorce rates have been increasing in the Western world for decades now (in the EU, for example, there's almost one divorce for every two marriages), and study after study has found that divorce makes women more vulnerable. This is often because women are almost always granted custody of children following the breakup of a relationship, and sole parenthood is really, really hard. It's also harder for women to be sole parents than for men, because they face much more discrimination, economic and otherwise. And there are plenty of other reasons too, but I've only got 600 words...
When marriage along these lines is applied to homosexuals, similar things start to happen: the partner with the lower economic power is made reliant on the partner with higher economic power, and then they get vulnerable, and if the relationship breaks up they're up shit creek without a paddle, if you'll excuse my turn of phrase. I don't understand why progressive homosexual people would want to put themselves and our community in that position. If anything, we should be fighting for the rights of ourselves and our heterosexual friends not to have to marry. We should be fighting for the benefits of marriage, such as tax breaks, couples' pensions, and so on, to be available without having to place one member of the partnership in a position of vulnerability.
So there are some good reasons not to get married, at least if you're a woman. If you're a man, it's probably a pretty good idea to get married, in that married (and even divorced) men tend to be better off financially than unmarried men do, but if you care at all about the person you're marrying, then you probably shouldn't.
There are plenty of other ways to celebrate your love for your partner: throw a huge party, have some kind of ceremony in a church if that's your thing, eat cake, go on holiday. All these things - the most enjoyable and, I think, most important aspects of getting married - can still happen without an actual marriage. Go for it!