Please, please, please, don't hate me for what I'm about to say.
There's decent evidence that penetrative sex was pretty rare outside of marriage prior to that shift. And there's better evidence that penetrative sex got a boost from a variety of people/sources after that shift. And there was also a shift in perceptions/beliefs/expectations about which gender was responsible for saying no/which gender was more highly sexed. That is: _men_ were expected to not stick their dick in anything, and women were believed to be more sexually aggressive than men prior to this shift and afterwards, things switched (women were supposed to say no, and men were more sexually aggressive).
Here's an idea (NOT Hitchcock's). We _know_ from what happened later on, that people get ideas about how to have sex from representations of other people having sex (porn). We _know_ that pornography was becoming more widely available during this time period. Maybe that had an influence?
The condom was "invented" (by Fallopio. Ha!) to try to slow/halt the spread of syphillis in the 1500s. After it had been around for a while, people noticed it worked as birth control also, especially if you doped one up with spermicide. By the time period in question, a variety of barrier/spermicidal methods of birth control were available in London, where earlier, about your only choice was herbal abortifacients (and anyone who has gotten pregnant and miscarried knows this isn't necessarily something you want to do a lot of voluntarily, even during the first few weeks).
Could the demographic shift have been a result of a combination of available depictions of penetrative sex (hey, it's in a book, it must be right and we should give it a try) plus the apparently effective birth control forms newly available (people suck at assessing rare events, so if you used a condom and it usually worked, you might tell all your friends before it failed)? That is, a perverse effect of new forms of birth control/"more" information available about sex?
That's why I said don't hate me. I _know_ that's what the rightwingers are saying now. And I hate their policy proposal (no information, no birth control -- because you cannot turn the clock back. Sex Ed is not the problematic source of info).
It does make me wonder, tho. If you really wanted everyone to be having a whole lot more fun and staying safe, returning to the yesteryear of lots of "outercourse" and no intercourse is clearly a great idea (supplemented by lots of prophylactic and birth control options), and one that some progressives try to promote, at least part of the time. And looking at what tends to happen whenever new, effective birth control is introduced (hey, we can skip foreplay and go straight to jamming it right in there now with the Pill because we don't need to worry about the condom breaking! Woohoo!) makes me wonder if this happened during the outercourse-condom transition. Especially since that's the exact same transition that decided the female orgasm was not necessary for conception.