Most people aren't even fully aware of this, but there were actually two sexual revolutions in the West. The first one happened in the 1920s, right after WW1, and was mostly confined to the wealthy urban socialite circles. Prior to this point, the dominant dating model embraced by virtually everyone was basically identical to what you see among the most highly religious people today. Not only no sex before marriage, but virtually no dating before marriage either, minimal time spent alone between prospective lovers, if they did go on "dates", there might be a married chaperone to ensure that nothing happened between them, and the first kiss was to be saved for the altar. Perhaps not everyone followed these instructions to a T, but they were still quite rigorously enforced and expected to be followed.

We all know what happens during a major war, and one of the consequences is a massive increase in both consensual and non-consensual sexual activity. The prevalence of sexual assault is well-documented, and on top of that many soldiers had consensual affairs with local women for a variety of reasons. Not surprisingly, STD rates increased dramatically. There's probably many reasons why extramarital and premarital affairs increase during war, but I suspect one mechanism is that people move from a long-term to a short-term survival strategy. When there is peace, and things are relatively safe and stable, people plan out their lives further in the future and that involves making careful sexual choices so as to avoid things like STDs and pregnancy out of wedlock. But when the future is unknown and uncertain due to war, some may adopt the position that since their survival is threatened, the long-term future may not really matter and may prioritize sexual pleasure in the now, causing a loosening of the rules that govern sexual behavior. Whatever the truth, the rules did become a bit more lax in the Roaring 20s and remained that way until the mid-60s. Young people were now allowed to "date" rather than merely seeing each other once or twice at most before marriage to see if they made an acceptable match. Dress codes for both men and women, while still conservative by modern standards, became a little bit more permissive as well. Since men and women were allowed to spend more time together alone, forming emotional connections during dating became more common and some physical contact was allowed, although it was mostly restricted to kissing and anything much further than that was still considered to be wrong. Dating was short and couples were expected to become engaged within a few months of dating to prevent things from becoming serious until the wedding night, or at worst mere weeks before when the couple were already engaged.

Even married couples were more conservative than today. For example, I remember reading that as late as the 50s and early 60s, many married couples considered it a sin to have sex with the lights on. My only guess is it was supposed to be done in the dark because sex was for reproduction and also the formation of an emotional bond, but that bond could be cheapened by admiring the physical body too much. Whatever the reason, this is certainly very conservative by today's standards. Even conservative people often think that sex may be forbidden outside marriage, but within marriage there are very few limitations, as long as both parties consent and both stay safe. But in the 50s this was not really the case.

Then you had WW2 and the same pattern repeated itself from WW1. Large increase in both consensual and non-consensual sex, and a corresponding increase in STDs, paternity fraud, etc. This time, the massive shift in sexual mores did not really occur until about 20 years after WW2, but changes did still occur as the culture had been becoming more permissive since the 1920s, and now they accelerated up until about 1968, when the hippie generation decided that almost all remaining restrictions on premarital sexuality should be thrown out the window. A catalyst for this was probably the war in Vietnam, since many young single men were now being conscripted to fight in another dangerous war that they could not avoid, and perhaps wanted to lose their virginity before dying or being seriously injured on the battlefield. Once again, short-term vs long-term survival strategy. Also during the 60s, up until the early 80s, the Cold War was in full swing, so even though no actual conflict occurred, people probably also adopted more short-term thinking, since the world could end at almost any moment. And as a result, the sexual revolution continued until the mid-80s, when rates of sexual activity began to peak and this was exactly when the Soviet Union began to decline before collapsing in 1991.

So essentially, the change in sexual mores in the West was fuelled by war and uncertainty and many of these changes persist to these day, without people being aware what caused them. What seems unreasonably conservative to many people today was considered pretty much the norm until 100 years ago, which is a very short period of time in terms of even the 5000-year history of modern civilization, never mind prehistoric times. There have been many other changes taking place during that time, many of them positive, but when war played such a clear role in accelerating a phenomenon as it did here, and when that phenomenon is associated with things like STDs, pregnancy out of wedlock, higher abortion rates, and a high rate of children being born out of wedlock, maybe it's time to reconsider whether the sexual revolution was really a good thing after all.