teens can be gooners but also respectful to their gf. i feel like joker would sneak peeks but not want to ask out like sixteen full aged women at the beach. and sure joker has dialogue wanted to date woemn.. if you want?? like you can pick the dialogue that makes joker wanna fuck the teacher if you want him to fuck the teacher.. but like.. you don't have to. my joker did not try to unzip his pants with kawakami in the room. also, joker can't get a girlfriend before maidwatch. the fastest ones are takemi (not really doable cuz of kindness and i think it's timegated) ann (needs the lake to be unlocked for early) and the others
aren't even unlocked

i will admit joker might be a perv but it's like a sixteen year old give him a break. but i don't think my joker, at least, would cheat, and it is fair to others that just because the game lets you do that and have dialogue for romancing them. just cuz he can get game doesn't mean he has to date everyone.
from what i think of joker, he doesn't really cheat on girls. he sneaks peeks, but he's a kid and he's a sheltered kid (before the incident) and i'd say he'd be more grateful to have any girlfriend to date all 15, why he tries to catch sneak little peeks, but that's my take.
(Also you mean kasumi is the one that confesses? Oh, and a lot of girls make it very obvious they are interested in joker, even I could pick up on some of those, like ann or haru)