My thesis is that humanity as a whole is becoming much better on average with ero. I mean just look at this:
https://exhentai.org/g/2768356/5075a68bfe/ Maybe I was just bad at finding stuff in the past, but I feel you'd be hard pressed to find this kind of quality in ero anywhere besides the best of the best in the past. Now it's far more common for artists to be on that level or higher, and there's so many you can't even name them all.
So the increase in groin shots might just be a side effect of more people noticing they're quite sexy when detailed well from the increase in exposure from the amount of good artists now out there, and they're trying to keep up.