I think their premise is that the Bible positions the Christian God as warning humans "Don't do it!" ("it" being homosexual relations) -- like a parent telling a kid, "Don't steal candy from the store!"

Those of us who engage in homosexual relations are supposedly disobeying his orders due to our "free will."

The fact that Bible passages may have been altered by self-serving human beings is a possibility they're willing to ignore/dismiss.

The fact that they may personally view homosexuality as "icky" just conveniently serves and boosts the predetermined theocratic narrative.