I'm not a native speaker, and may miss some nuances, but promiscuous is defined as "having multiple sexual partners" by my dictionary which I think is as neutral as possible.
Promiscuity is at first, and foremost was (and still is, in spite of contraception availability) practically more acceptable for men. And monogomy among other things is a constraint on men's behavior - forcing to accept social, and economic responsibilities before fulfilling sexual urges. You are right noting that it rarely worked in any absolute way, but I don't think any law, whether written, or not can be 100% effectively implemented. People steal all the time, but I suppose you wouldn't say it's because related norms are not enforced, right? Monogomy's real application varied over times, cultures, and spaces, but it was certainly enforced by societal pressure, and legally too. Actually, in my father's youth years (and it's not that deep in history) it was hard to have sex before marriage. It was for sure possible, but dangerous for reputation of both participants (probably more damaging for a women, but damaging for both nevertheless). It wasn't US, and "sexual revolution" came to us much later, so that's probably why you may have no living witnesses for it.
Also, prostitution was until quite recent times seen as a moral compromise acceptable only because of long serving soldiers, and sailors. In most societies it wasn't ok for a family man to visit a house of prostitution, and in some cases it was even criminalized.
