When I see liberal Christians saying Christians needn’t be anti-gay, they often seem to use two conflicting arguments: the Bible doesn’t say what most Evangelicals think it says, and anyway the Biblical authors didn’t know any better. Example: this Hemant interview of Candace Chellow-Hodge. Not sure if this quite belongs in the “stupidity” category, but still very odd. Note that I’m pretty sure there are some Christians who take Biblical authority seriously and seriously believe the Bible doesn’t condemn homosexuality, but they seem to be a minority of those who deal in pro-gay interpretations.
I think it’s just a pragmatic thing. There is a powerful argument to be made that you shouldn’t take the Biblical authors as infallible. But even if an evangelical refuses to budge on that point, it’s helpful to have a backup argument that is compatible with infallibility.