Having premarital sex is not everyone's definition of "promiscuous".
I'm merely observing a lot of factors which in aggregate can unquestionably be seen as causing this.
The reality is that the traditional gender roles where very positive in the context of reproduction, which was literally my first sentence of my first comment.
It is not a judgement on wherever we should aim to revert to them, it's just factual. Arguing against that is basically at a level of arguing that water isn't wet.
Now to link this back to the discussion at hand: a significant chunk of society would consider premarital sex with people whom they aren't planning to marry to be promiscuous. And those people are part of the population which wouldve become families in a different age.
Birth rates going down seems to be a thing. That's about all I agree are facts here. I struggle to even meet you at "traditional gender roles" like that's some universal constant - is that Protestant America? Catholic Ireland? Is that one of the Chinese dynasties? Sub-saharan African tribal society?
I think, like most things, it's unlikely you've found the "as obvious as water being wet" single smoking gun to a broader solution.
Social pressure to marry young and breed will clearly have an effect on birth rates. I'd be surprised if anyone would disagree there, all other things being equal. It feels ridiculous to assert that is the only possible influence and even more ridiculous to assert one particular set of social norms is the only way back. I know so many people that don't fit this incredibly narrow view, including everything from "traditional" couples not wanting kids (for lots of different reasons from money to global stability to being jaded to genuinely not caring) to very very not "traditional" people who ARE having kids.
If this is worth talking about I think it's worth taking in more info than just blaming resentment over women being more empowered over their own lives (or more slutty or more undesirable or however you want to frame it).