Bill Barr asked an interesting question in the comments.
The deeper question though, and the one that gets to your buiding UUism, is why a faith that welcomes same-sex marriage has so little appeal --it seems to me-- to Gays?
I lived in Oak Park Illinois with the largest per capita population of Gays in Illinois yet I always felt the conservative and Catholic Churches had far larger Gay memberships. I'd ask Gay friends why and it was because Christian faith that appealed to them.
Our's didn't. Something was missing.
Because gay people pick churches based on faith, not on the church’s politics. Why shouldn’t they? Everyone else does.
This is one more reason why we should focus on getting our message out theologically and growing the church rather than telling the government how to run. Because if a person, gay or otherwise, does decide to visit a UU church, and his/her first Sunday the sermon is about the importance of forcing congress to put someone we like on the Supreme Court (something the UUA Washington office encouraged ministers to preach about,) that person is going to assume we are the League of Women voters and tell his/her friends that and anyone who cares about spiritual concerns will stay away.
Honestly, would you rather be not particularly welcome in a church that you felt would otherwise nourish you spiritually? Or going someplace that accepted, and to some degree pandered to, your demographic, but didn’t feed your soul?
For me, at least, that’s not a hard question.
I honestly think that lots of people, gay and otherwise, would join UUism if they knew who we were and what we believed spiritually. So why don't we talk a little less about how we know how the world should be run, and a little more about that?
Make it not suck, and they will come.