I get really irritated when people use the name of God and hide behind religion to discriminate against others. Anyone who uses religion to bash others is an extremist and has issues with themselves. If these extremists/bible literalists want to go back to living as they lived 3000 years ago then they can separate themselves from the rest of society and create their own communities. But they should leave everyone else out of it.
You can tell him there are believers in Judeo-Christian tradition who disagree with him and are extremely ashamed of being surrounded by ignorant literalists and hypocrites. I see the bible as ancient people's understanding and relationship with God, not to be taken literally because society evolves and we are not like we were 3000 years ago. If he wants to be literal he should go out stoning people. Society evolved, that same religion evolved too (see how Reform Judaism in the US made its own rules to protect trans and gay people in their communities). If he's a Christian he has no business talking about Leviticus or whatever he is referring to. I am an ex Christian, I came to hold different beliefs now but I know how they distort and cherry pick stuff. I know them very well.
But there is no point in arguing with people like that. It's a lost battle unless they change and are open to education. What is in his mind was openly debunked by serious, unbiased science and if he's too arrogant (or maybe scared) to educate himself then let him drown in his own sea of ignorance.
You say his side has lost the war. I really hope so. Unfortunately, I tend to be pessimistic about these things. Maybe because I live in a rural town somewhere in Europe where we don't even have equal marriage.
Maybe when I get hrt I will relax but the reason why I can't immediately start hrt is because of people like that around me. So it makes me angry.