I probably would. Paul attended the services of people who no longer followed God, so I don't see any harm in doing so. My presence does not equal my condoning of the actions any more than Jesus's presence at the wedding equaled His condoning of the drinking going on. (and no, I don't think turning water into wine was condoning the drinking, either).
Gay marriage is wrong, as is homosexuality in general, but if a friend of mine who is a homosexual asked me to go to a wedding, I would lovingly accept. I would hope that perhaps I could be used by God while I'm there.