Leviticus 18, which includes such sin as approaching a woman sexually while she is on her period, is indeed a list of sexual laws. It is part of a book on laws and is sandwiched between the chapters containing all the other prohibitions I mentioned. Choosing to accept one chapter of that book, but not the ones before or after it makes very little sense to me. Picking and choosing between chapters makes as little sense as picking and choosing between individual laws.