The starting point was an absence of sex in the public domain, an avoidance of reference to basic reproduction facts, a ban on anything beyond, etc.
Things have progressed to sex scenes becoming so prevelant and readily accessible that it's become desirable to have a choice to see material that doesn't constantly interupt plot for steam.
Sex has moved from titalling rarity to boring omnipresence. and is now winding back somewhat; this is not a backtrack from the state of play in the 60s though, the world has leapfrogged past that era.
The era you're calling back to to was just a hard push back against a post-war surge in clean imagery and buttoned up moralizing, amplified by the Hollywood Code, but you need only go back to the early forties to see and hear wide open celebrations of tawdry lives, and can dig back farther again to see repressive times around the Prohibition era, which you might reasonably assumed was in response to a time that was not so repressive at all, etc etc
In many areas, we can see the incoming generations being the new teetotalars and moralizers eager to temper things down again.
As for the role of feminism, it's found a voice on both sides all along, during all of the cycles. That's its reflection of true freedom. Even during the "Free Love" era and its direct continuance, the Gloria Steinem voices were driving a different vision than the ones celebrating "free love". And in fact, you can see a through line of that tradition within feminism to the one you're critiquing in the present.
There are still patriarchal abstinence-only programs in a considerable part of the US; so that can be affecting attitudes some. But also; with increased awareness of queer and asexual personalities and relationships, and emphasis on consent in general; that can be overall influencing attitudes as well.
In a world where teens are a click away from the referents to "Ma'am I do my own plumbing" and "Wh-what are you doing, steppe peoples?", I'd guess they don't see sex as negative so much as see gratuitous sex, sprinkled in by writers who can't even quote a single midget donkey porn trope properly, as negative.
We pay about $60 for an evening for 2 at the movies; for that I'd appreciate having 90-120 minutes of, you know, story.
