I think that the amount of sex has been at a relative constant since the 1970s. It went up around then (free love, as others have said) but I don’t think people have stopped screwing in the last 40 years.
My guess is that it’s a combination of the following:
- women (and men) started discussing things more, and being safer about STDs, before they have sex. In other words, women started speaking up for themselves.
- AIDS scare (although that wasn’t seen as a serious threat until the 80s)
- Condom availability – it was no longer considered embarrassing for a man (or woman) to by condoms, so they did, and used them.
I do think that @Imadethisupwithnoforethought ‘s friend is correct people are still sleeping with anyone they want, but they’re smarter about it.