I don’t think they’d even need to go that far.
There are already churches out there which are happy to celebrate homosexuality. What you’re going to see is those churches being pushed more and more as the “mainstream” face of Christianity, with churches which object to this or refuse to allow same sex marriages to be conducted on their property being seen increasingly as dinosaurs incapable of “moving on” with a “progressive” society.
Sooner or later you will see Hollywood and television and the rest of the media presenting an America where the mainstream Christian churches are the likes of the Metropolitan Church and other gay-friendly groups, and those who attend older, more traditional churches will be seen as either oldsters who can’t keep up with the rest of us or kooks.
I think you've underestimated the left, especially the sodomite-loving left, they'll not be satisfied to have 80% of so-called "mainstream" churches performing gay rites, so-called gay marriages.
The atheist gay-loving left will not leave the 20% dissenting Christian Churches (note not all faiths, nope, no Muslim Mosque will ever be forced to accept a gay in their midst, let alone perform rites other than beheading) alone, dare I say, when the 20% of dissenters become 1%, the left will ultimately force them to accept sodomites or shut down and become outlawed!