Because it promotes social stability ? As much as I dislike defending religion - those values produced the most stable societies through history
Societies of n>100s. By tabooing sex you reduce promiscuous behaviour - which stabilises society. I don't really see how this would be controversial. Modern social values have unambiguously shown that they lead to a population decline. Huge difference being that technology makes us less reliant on population count for stability (hopefully).
I don’t actually think the answers to these questions disprove your statement, because I have a painful lack of knowledge as to what those answers actually are. But I do feel that those answers need to be given before an argument such as yours can make sense.
(Granted, a trait that promotes societal stability can become common even if stability isn’t actually good, so the last question is not as important as the others. A dystopian equilibrium is still an equilibrium.)