if he just watches, it’s harmless. maybe annoying for you, if you dislike being watched. you don’t know his intentions. maybe he was just getting closer to cheer or something.

also, wanting to have sex is on it’s own also harmless and natural. doesn’t mean he’s a rapist.

i think his actions do not necessarily call for a reaction. just, as you also felt uncomfortable, you did the right thing. it’s public space, so even if it’s rude, everyone can stare at whomever they want. you always risk being exposed to rude people when you do things in public. sometimes the best thing you can do is just leave. But if you’re not alone, you can call him out to find out more about his intentions. Maybe he’s chill and will change his behaviour once he’s aware it makes others feel uncomfortable or even threatened.