Matthew, I don't know if this speaks directly to your point or not, but though I do not favor a theocracy, in which church and state are united, in my view Biblical principles still must interface with our social institutions, and part of the church's role, individually and corporately, is as a prophetic voice to the culture.
The root issue is whether we remain a nation that honors God. We certainly did
when we were founded. Congress opened with a three hour prayer meeting; its
first major legislation, the Northwest Ordinance, authorized using the Bible as
a textbook in public schools. There was a wall of separation between church and
state, in that there was no official denomination (thank God - though at the
time there still were official denominations in some of the individual states),
but religion had a honored, preeminent place in the public square.
If I didn't have the word I can see how the humanist argument would sound good.
But the word and the Spirit give discernment that enables us to separate loving
the sinner from hating the sin, something the world can't understand.
Unfortunately, we as a nation have been choosing to abandon God, if not spit in
His face. There are consequences to that, and they are not going to be fun. But
I believe that the challenge they will present will be an opportunity for great
revival in the church.
By the way, every one of those parallels you draw in your initial post (incest,
bestiality, pederasty, etc) have their advocates among humanist leaders. It goes
all the way back to the Margarete Sanger crowd, and includes Kinsey with his
fraudulent reports on human sexuality.