Might have something to do with movies catering more heavily to "Family" audiences (the functional death of the R-rate film in theaters) while TV shows cater more heavily to childless adults.
We've literally coined the term "sexpository" to justify injecting gratuitous naked time into our 8-12 hour prestige TV shows. We've also made these sex scenes a lot gayer, which rubs the Family Friendly crowd the wrong way but plays great with the 20-30 somethings who are all on board with it.
I think this is it. Most movies are trying to avoid an R rating and slide into PG-13, because PG-13 movies generally make a lot more money than R-rated ones because the audience is greater.
Agreed, well placed and executed action or sex scenes can be used to great effect. They are also equally capable of adding nothing but wasted time. My point is that bad action scenes have been wasting plenty of time in movies lately, so a desire for tight films with no fat is definitely not the cause for the comparative dearth of sex scenes.
Nah if you want to watch porn you can just watch porn.
Access to adult content is never been easier so we don't need half-assed versions in our film.
also, there's some selection bias as the "top X grossing films" has included more and more childrens films over time as adult consumption of all content has moved out of the theatre and to the internet.