It is observably a fact that the "sexual revolution" was a qualitative change and not just a quantitative change. Presumably something changed for that to happen. A student of history will note many, many times in the past that cultures would have been willing to make that change, but couldn't for some reason, so "it's just because we moderns are so uniquely awesome and wise and wonderful" doesn't strike me as a likely answer.

I suppose it could conceivably just be the ability to treat STDs, but I don't think that's enough, personally. YMMV. (Plus I'm not sure that the time works out correctly.)