There are some things which sound good at the time, but many generations of experience have shown that they don't work.That was a comment made by my mother when I was a teenager, with a veiled reference to sexual morality. There is a tendency is some circles, I have noticed, for what is termed "traditional sexual morality" to be regarded as some collection of irrational taboos, or at least something no longer applicable to modern circumstances. Sometimes it is called "Christian sexual morality", but that is a misnomer. It would be better to call it "human sexual morality", for it is the basic default system from which individual cultures tend to deviate. What Christianity introduced is a add-on: the idea that what is sauce for the goose is sauce for the gander. Committed as it is to sexual equality, Christianity has always been negative towards polygamy, divorce, and the major exception to the default system practised by civilisations: the idea that it is acceptable to have a pool of degraded, low status women ie prostitutes, as an outlet for otherwise "respectable" men. However, the desirability of chastity before marriage and fidelity afterwards is the general rule of all human societies.
Some cultures, believe it or not, have sexual moral standards stricter than ours used to be, and attempt to restrict nearly all communications between the sexes. Where their standards are slacker, it usually means carving out exceptions to the general rule - exceptions which, as the proverb explains, imply that the rule exists. The unusual exceptions - the ones which titillate anthropologists - tend to disappear when we leave the small tribal societies and examine the major civilisations, and the reason is obvious. These are the successful societies; they have been around a long time, and each occupy a large section of the world's area and population. They have discovered, as we are now having to relearn, that if you don't keep sexual relations within the bounds of marriage, things start to fall apart.
