When I used the dev mode to re-add Gay trait to both affected pawns, they immediately got back to doing the dirty the same night.
I encountered the same gay-couple-not-having-sex problem, reverted rimworld back to a17b and used only hugslib and a17 psychology, the gay couple do lovin' every other day. I used the devtool to check the lovin' MTB of the couple, here's the results:
Anya (female) has the lovin' MTB value of 36,5h with her spouse, Kimmy (female). Both pawns have max sex and romantic drive and kinsey scale of 6.
When I made a gay couple in b18, using only hugslib and b18 psychology, I got:
Mumz (female) has the lovin' MTB value of 1614,3h with her spouse, The Nose (female). Both pawns also have max sex and romantic drive and kinsey scale of 6. On the other hand, Mumz has the lovin' MTB value of 30,8 with Hammer (male). Hammer also has max sex and romantic drive, but has kinsey scale of 0.