What is the fundamental reason to why Abrahamic religions are against sex?
Abrahamic religions are absolutely not opposed to sex. We got ourselves, unfortunately, a stiff & stuffy reputation, but that doesn’t mean that’s what was laid out. You’re right, sex does have a significant place in Bible. I think the biggest reason we have that rep is that God’s definition of anything isn’t going to be our definition, purity included. His expectation of sex is to be in marriage, & there it serves as a bit of an illustration for how He wants us to be faithful only to Him, just like He expects us to be faithful to our spouse (see prophets, especially major prophets, for imagery here). Sex is expected for Godly couples. Late in the New Testament, Paul wrote to the Corinthian Christians that they couldn’t “deprive each other except by mutual consent & only to pray about something specifically together.
