Regarding sex in most mainstream anime:
-protagonists are young, ranging from preteen to 17 most of the time.
-Older protagonists are mostly out of your average person's life/behaviour
Maybe that's what I watch/like that is reflected, but it's not a normal with these settings to rarely have sex scenes even implied.

But you still get dirty jokes, you get some gaps created by the frustration, and sexy girls make better sales.

As for tv rules, well a studio can find story plots, but my guess is it's not worth it most of the time. Too much effort and no return or even drops sales.
I know SAO. But they told us she's 21 (at least from memory), probably for a reason and also for sales. But just my guess.
Domestic na kanojo is different in that sex is first ep and almost never after that. No problems for sales as the story was a success with that start