When I’m in doubt, I always go back to the two commandments. Yes, TWO, the ones Christ gave us. So I think sex is a sin if it means you are hurting somebody or living in a lie - because God is Truth, as well as Love. So a lot of normal, heterossexual, marital sex is a great sin and some pre-marital sex is not a sin at all. (For instance, a man who rapes his wife is a sinner, a man and a woman who can not be legaly married because they lost their id and have sex because they are absolutly sure they will marry are not sinners).
To all the people who asked what sex is for: it’s for making babies AND having pleasure. Why else would God create the clitoris? It’s only function is to make sure women who have their vaginas hurt by childbirth can still enjoy sex, just like men do. God is not a male chauvinist!