Okay, so addressing Lev 18:19 - so… Depending on where you look or what you read, modern Christianity has more of a general, practical soft taboo because it’s messy and “she’s tired”, and depending on what you’re reading from where, some people think she should still service her husband manually or orally while others are like, “umm… No.” But nobody really brings up Leviticus 18:19 as a hard prohibition. There’s a reason as to why.
So, I’m going to start by saying some pockets of Christianity do follow more of the levitical law than others, and there is wiggle room on certain things for interpretation. However - the general rule of thumb is, if an Old Testament/Mosaic Law is repeated or upheld in the New Testament, then it stands as a hard and fast rule. If it is directly repealed (like the dietary restrictions and various sacrifices) then it’s obviously void. If it isn’t directly repeated or voided in some way then it’s up for debate and individual conviction.
So, for example, a lot of, if not most, of Christian opinion is that the laundry list of random things that make you unclean are voided with the resurrection of Jesus because once you’re saved, you’re “clean” before God permanently. Because period sex isn’t brought up again in the New Testament directly, it’s kinda fallen into the “you probably shouldn’t because it’s messy and icky, but none of the Apostles say it’s a sin, so… As the Holy Spirit convicts.” And I have heard some people say it’s wrong, but I’ve also heard others say it’s okay. It’s up for interpretation.
Homosexuality, beastiality, and various forms of incest and adultery are addressed or mentioned specifically in various Epistles as a no-no, therefore cementing the prohibition going forward.
Does that make any sense? Certain aspects of theological study border on legal interpretation and others on metaphysics and orders on philosophy. If a person wants to make a study of it, it’ll certainly exercise the mind!