Because gays and lesbians opposite-sex marry to please their religious, conservative parents, then have children, then split with the opposite-sex spouse, and then finally end up raising those children with same-sex partners. Oh, and by that time they also either can't leave because of child custody agreements or else don't want to leave because it's good for kids to be near their extended families and other parent.
Of the few gay Mormons I have met, none have left the church. They are proud of their heritage no mater how fake it is.
They still think they are better then everyone else. I have heard comments like "cheating on their wife....Christians are always doing that"
I have not met that many, but of the few I have they bare a striking ability to compartmentalize things in a way that the two worlds collide.
When Mitt Romney ran for president, it was shocking how close their though process was to his.