It's... kinda complicated, I guess. Part of it, I think, is that between decreased tolerance for bullying and the support of friends met through social media and such, there's not as much stigma or shame attached to being a geek (or for that matter, to being openly queer or sexually active-- the last especially for girls.) Another big thing is that having a few high-profile movies, TV shows, and things like that come out in the last decade or so has given audiences that wouldn't normally have gotten into geek culture a taste of it-- and lots of them are finding things to enjoy in it. Before the only way to really get into the Avengers was to get into the comics-- a niche market with a lot of continuity to wade through, so not very accessible. But now it's a tentpole summer blockbuster franchise, which is much easier to get into, especially if the trailer ran with a movie you were going to see anyway and hooked you, or a celebrity you follow is a part of it, or if because of my first point you have an openly geeky friend willing to help you get interested. Thirdly, things are still tough all over for millenials (I myself went three years living in a medium-sized city without getting a job interview despite applying all around town) and naturally they gravitate to fantasy (in the general sense, not specifically the fantasy genre) and nostalgia as forms of escapism to help them cope with how crappy a lot they've been dealt. Lastly, I think Harry Potter is a big factor too-- between the books and the movies, Harry Potter was a huge franchise for like fifteen years. If you started with the first book when you were 7 like I did, you would be in your high school by the time the last one came out and in your early 20s by the time the last movie did. If one franchise can follow you from childhood all the way to adulthood and retain its charm, the reasoning might go, why not other things?

As to sex... that's even more complicated, but the short version is, I think, that society has mostly become more open and comfortable with it in general. This has good effects (if you are a girl and sexually active as a teenager now, you don't face nearly as much stigma as once, for instance) and bad ones (at the same time, there's more pressure to have sex now.)

And as you say, at least part of the population paid lip service to chastity-- for how many of them was it sincere?

I dunno, I'm probably oversimplifying a lot, but I'm not feeling great today and I figured you deserved at least a token answer. Let me know if there's anything I can clarify.