In my experience straight people do a bunch of things on the way, as well. So I think it's less about not realizing that people change positions and more of a fictional construct so you don't have to write all that action. As when people read smut they're often like, okay, come already, jeez.

But could you do like, a ton ton of assplay on someone from that position? Like, the guy is getting seriously fingered and stuff in this story, so that bit doesn't scan for me, either.