I can't really answer the OP. I agree it would be good to go and share Christ with people. But on the other hand, a gay couple is bound to have a lot of gay friends, and these days that often means militant radicals for their cause.
So if I went, would I actually be able to witness, or would I end up in a big fight? Or keep my mouth shut and endure everything that was going on?? As for whether it would be condoning it, I am not sure about that either. They are going to get married with or without my approval. So if I say no, do they care? If I say yes, will they take that as me agreeing with them, especially if I have made it clear that I am a born again Christian who believes the Bible?
I don't know any gay people, which is kind of sad, in a way. Not that I approve, but that how will we reach the lost, if we don't know them?
Good OP, I've been thinking about what my answer would be since I read it, earlier today.