When you get to the end of a sex scene (first time viewing only for quest-related ones), there's an experience bar that pops up in the lower left of the screen, no? Then again, I have a later version of the game (I'm a Patreon sub), so maybe it's not in 3.28, in which case IDK.is there a way to tell what rank it is?
In any case, I don't think anything is tied to sex experience yet—it's just a mechanic they're planning on doing something with later.