Why does my cat do this?

Our two cats aren't affectionate to one another, and are just tolerant. Our female cat (the one we've had longer) only recently got used to our boy cat (we got him last year). They're able to share the same general space, but our girl cat does get a bit hissy when the boy cat pops out of nowhere, but that's the extent of the aggression. The boy cat is pretty chill, and brushes everything off pretty easily.

Recently, I saw them both go into the room with the litterbox. I thought they were going to get into a fight if they were in such a confined space, so I followed them. As I peeked around the corner, I saw the female cat peeing, and the boy cat just staring at her pee (the girl cat had her back turned to him). After she was done, she jumped out and walked away. The boy cat didn't even pee afterwards, he just walked out also.

I joked with my partner that we adopted a pervert of a cat who likes to watch girl cats pee, but I was wondering what the actual reason for that was. He knows that she doesn't like it when he's all up in her space, but he still got pretty intimately close, and she didn't even react.

I honestly can't figure out cats sometimes, so if anyone could explain this behaviour to me, that would be super!