if the building has been dedicated to God, how could they turn around and sell it?
As dedicated witnesses of Jehovah, we still must "sell" our skills and labor to an employer to provide for everyday necessities. Similarly, what a wonderful opportunity we have to support kingdom interests by renting out dedicated buildings in order to fund the preaching work!
Doesn't take much to twist things in order to justify it. The civil rights issues could probably be side-stepped in most cases too. For one thing, I can't see any gays wanting to get married in a KH. JWs only do it because they can't afford a nicer place due to the education ban or the fact that they mostly get married right out of highschool. what in the world (other than having it out for the cult) would motivate some gays to want to get married there? Even in that case, can't churches establish rules about who can get married there? Things like pre-marital counseling, etc? They could probably make it either prohibitively expensive for a non-JW to get married there, or set the rules so strict that no one would bother. They could count all the meetings, etc that JWs do as time towards this requirement and therefore JWs could still use it to get married without the trouble.
Anyway, I suspect they'd be able to come up with something. If not, they can always quote a few scriptures out of context to silence any critics.