Well, it's a hard question. If you're personally performing the wedding ceremony, that's pretty obviously a sin. And naively believing a politician is not a sin. Voting for a politician who supports gay marriage but doesn't make it a central issue of his campaign strikes me as pretty benign, because the other issues may make him a better candidate so that the ends justify the means, but this requires thought and different people will likely conclude different things. By contrast, voting for a candidate who supports abortion strikes me as much more likely to be sinful because it is like you are an accessory to murder. It would require a lot more to justify, if that is even possible.
On the other hand, there is probably a lot more personal hatred and fear towards gays in this country now than towards abortions, so gay marriage may draw out conservative voters to the polls even more than abortion. And that might also make sense given that not voting against the candidate who supports gay marriage is probably not a sin. The Catholic Church says sins of omission are just as bad, but the prevailing trend in modern American conservative thought is to make a huge distinction between action and inaction.
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