You're most likely to find such people among principled libertarians. By their very nature they:

    1) Don't care what consenting individuals do.
    2) Want the power and influence of the State reduced.

Some of us are reasonably pragmatic and acknowledge that the State isn't going away any time soon. Therefore, we prefer that the State be as egalitarian as possible.

Others are less tolerable of pragmatism and do not support any expansion of State power. This has absolutely nothing to do with sexual orientation and is easily applied consistently.

Moreover, libertarians are very unlikely to even acknowledge marriage as a right. It's only meaningful in our society as a "right" precisely because governments grant special privileges to married couples. This discriminates against ALL unmarried people---not just gays.

If governments weren't involved in marriage, ALL of this would be a non-issue. People could choose to celebrate or signify their union in whichever manner they choose. To an anti-State ideologue, claiming that "well governments are involved so you might as just give them more power" is just a non-starter.

I get that participating in a lynch mob can be fun. But my only point here is that there are legitimate arguments for the other side that don't require bigotry. (Since other commenters were claiming this to be impossible.) This is an inconvenient fact for a lynch mob acting on limited information.