In particular, the idea of very strong and widespread norms against sex outside of marriage is something that is hard to actually conceive of.
Progressives find the idea repugnant, and can't imagine why anyone would ever have supported it.
Conservatives and reactionaries can be on board with the idea, but still, it actually stretches the imagination to think of what it would be like for everyone in Europe to agree with the idea.
But this is mostly a failure of imagination, albeit an understandable one.
What would be the minimum number of changes necessary in society that would reverse the change entirely?
You could rout all the current progressive institutions, and replace them with Islam, or the Catholic Church of 100 years ago, but these are not really minimalist changes. We want a societal Rube Goldberg machine, where we set off small changes somewhere else that get us the same outcome.
There's an assumption buried there that the change might be reversible, of course, and perhaps it isn't.
But if it is, a good starting point is the set of things that might explain why the old regime got replaced by the new.
My suggestion - to understand pre 20th Century sexual morality, all you need to do is imagine a world without any good contraceptives, abortion, or birth control in general.
Which, by the way, was what it was like.
You can talk about the pullout method, or the rhythm method. But do you think these are going to be reliable for a teenage boy having a dalliance for the first time with a maid? Probably not.
And as soon as you do that, suddenly everything becomes obvious.
Take away contraceptives, and sex leads to pregnancy with high likelihood. Take away reliable abortion, and everyone, rich or poor, has to deal with the the child. Take away modern wealth levels and the welfare state, and an unplanned child for a single woman is a catastrophe.
How would you, enlightened progressive, feel about your 14 year old daughter sleeping with her boyfriend if it meant a good chance of getting pregnant and needing to have the child?
Suddenly the patriarchy doesn't seem like such a silly idea now, does it? Suddenly 'sex positive' messages to teenagers don't seem like society's number one priority, no?
But to reactionaries, the depressing flip side is also true.
Namely, if the absence of birth control was the the basis for monogamy and chastity before marriage as social norms, it's probably going to be quite hard to put that toothpaste back in the tube. You can't uninvent condoms or the pill.
This is like mass immigration - a social problem that's really a technological problem.
So I predict that our current sexual free-for-all will go on at least until society degenerates to the point that it can't produce contraceptives anymore, at which point barbarism will restore chastity before marriage.
On the plus side, when this happens, it will also simultaneously solve the most difficult problem of our times, convincing rich, educated, civilised people to have more children.
Give people the choice, and they will hack their own evolutionary reward systems and have a lot more sex and a lot fewer children.
Like Prometheus, we have stolen fire from the gods.
Like Prometheus, we cannot give it back.