I oppose same sex marriage because:
A) It reinforces heteronormative conventions in a way which is harmful to the broader queer/trans community, and even to the acceptance of alternative hetero lifestyles. Instead of helping us move away from insistence on (and assumption of) conventions like marriage, nuclear families, and the cookie-cutter heteronormative lifestyle, gay-marriage reinforces it. It reinforces the idea that pre-marriage relationships are judged by whether they are moving towards marriage. Marriage by whether its moving towards having a family. Career, marriage, family, church, little-league... Look at how most gays are represented on TV shows, they are always excessively heteronormal suburban couples. Their lifestyle is otherwise identical to hetero couples (probably even more vanilla that the central family), their issues are always mundane hetero issues (like trying to adopt kids, or buy a home), with only a token issue of "gaining acceptance" thrown in for flavour.
(Indeed, in my opinion, the religious nutters should be on the front-lines of supporting gay-marriage. It reinforces their narrow views on acceptable lifestyle.)
B) When legislators and institutions had to deal with the issue of gay couples without marriage, they tended to also look a multiple potential alternative forms of relationships, and across a whole array of social/legal institutions. And that helps people who aren't just simple gay couples, but who deal with the same issues in employment/taxation/pension/inheritance/custody/medical/etc. But when legislators can just say, "Okay, this subsection of non-heteros will be made honorary heteros for legal purposes", suddenly that allows them to ignore the same issues that affect other types of relationships (including couples who don't marry. "My partner and I have a lot of trouble with the way laws/regs are written" can be dismissed with "Well, you have the choice to get married. If you refused to do that, you only have yourselves to blame if it causes problems.") Gay marriage hides a lot of these bigger issues under the rug.
[Note, this includes issues that affect mundane heteros. Issues like estrangement (where your legal partner isn't the person you currently live with), single parents, serial relationships, mixed families, surrogacy rights, non-parent custody rights, etc etc etc. Gay marriage allows legislators, employers, institutions, to go back to pretending that people get married early and once, have identical family structures, and live together until one of them dies.]
C) It peels away the largest and most hetero-acceptable portion of the GLBTTI community: gay couples. Therefore vastly reduce the visibility and power of the queer community, reducing any future progress on issues that matter to non-heteros and alternative lifestyle heteros. The mundane heteros could identify with simple gay couples, with jobs and homes and otherwise identical lives to their own. That served as a "ice-breaker" to gain broad support for social reforms. Without that common element, and with gay couples encouraged to adopt more and more heteronormative lifestyles, it reduces the ability to the rest of the community to pursue social reforms and address social repression.