> 80% reliable contraception is of dubious utility.

You realise the quoted reliability rates are per year, not per act? For example, the withdrawal method (available to everyone, for all time) has a reliability rate of 78% for typical use and 96% for perfect use. [1] Obviously, even at 78% reliability over a year, that is of a great deal of utility. 78% of the time a woman whose partner practises withdrawal will not get pregnant over the course of a year.

[1] https://en.wikipedia.org/wiki/Coitus_interruptus

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.)

Birth control pills have a 9% failure rate, and anyone who’s taken it will tell you that the experience is far from “without consequence.”

I didn't say "without consequence". I said "safe". And by "safe" I didn't mean "literally no-one ever will ever have a problem with it in any way", but something more like a normal meaning of the term.