I would say this is more of the result of social progression. We have very strange contradicting and hypocritical laws or expectations to follow. Somehow it’s inappropriate for people to be seen fucking or nude yet it’s something in any other species we consider natural and celebrated. I get turned on by more casual shit like two best friends pissing next to each other at the urinal or seeing a guy show his asshole as a joke. These are things that realistically shouldn’t be attractive but there’s a certain intimacy about the platonic normality of these situations that can be waaaaay hotter than actual full blown fucking.
If we were more open and less critical we might not have this weird thing where this kind of “normal” thing is such a turn on.
Specifically though when we’re just talking about seeing a dick, most of the horniness comes from knowing a canon cock size/colour/curve/intact etc. it adds more human depth especially to a male where it really is luck of the draw and comparisons etc are a huuuge fetish/turn on.
TLDR overall I think the most basic human shit is attractive because it is just plain human interaction and society is just autistic as fuck and lacks this kind of basic intimacy.