I think it's the opposite. I think the adults are watching series that have more complicated plots, and require a longer attention span. They're taking their kids to go see movies like Thor and Transformers where there might be sexuality (Megan Fox doing pretty much anything) but no one is actually getting it on. Part of this is almost certainly large movie studios not wanting to push the bounds of conservatism, but it might also be that actors nowadays are less willing to get their kit off.
Of course, my head canon is that Neo and Trinity killed off the movie sex scene in their Zion romp in The Matrix Reloaded. I can't put my finger on what it is that makes it so offputting, but it makes tortoise sex look hot by comparison.
That's actually just about what I was thinking though, not really the opposite.
I guess I should have said that it isn't the horny teenagers going to movies on date nights by themselves these days. It is more Mom, Dad and the kids.
I can remember the "zion rave scene" in Reloaded, but I didn't remember they were getting it on at all. Must have just blanked that entirely out of my memory.
The Matrix scene just wasn't necessary, and went on too long. And was intercut with a raunchy-music-video dancing scene. It was a bit odd all round, especially in the generally clinical, straight to the point world of the Matrix movies.
Of course, my head canon is that Neo and Trinity killed off the movie sex scene in their Zion romp in The Matrix Reloaded. I can't put my finger on what it is that makes it so offputting, but it makes tortoise sex look hot by comparison.