we never see eye to eye on boundaries

i am my boyfriends first real relationship, we’ve been together for almost two years. he has mostly girl friends and in the beginning especially we had a lot of boundary issues (being extremely too friendly or even being flirtatious with compliments) i hold a high standard for respect in a relationship and shut down that previous mess immediately stating he better pull it together now cause there will be no second chance. it’s been a while since we’ve ran into a boundary issue but of course when he went back to school his neighbors just so happened to be a group of girls who i noticed were obviously being flirty with him and his roommates when they all first moved in. i always trust him to not do anything bad, my worries always lie in him not understand what is crossing a boundary because he doesn’t view it in such a way (even after we’ve communicated i still worry). since it’s been a while, everyone from both apartments have made friends and they hang out occasionally which i am fine with. tonight he told me he was going over to the girls apartment with one of his roommates to which i said okay, this was at about 12am. going on 3:30am he is still there so i told him i think it’s time to head back. he’s always said how he doesn’t see how the time of day matters when hanging out with his girl friends. i tell him here are appropriate and inappropriate times to do things with your girl friends to which he doesn’t understand. he said they’re just conversing and that late night talks are normal to which i said i see them as something you would do with your partner or very close friends, not your flirty neighbors (i just think it’s a bit too comfortable). it’s so frustrating having to explain why certain things make me uncomfortable because he always likes to say “i wouldn’t care if you did it”. which is easy for him to say since i have literally never done anything even remotely close to making him feel uncomfortable about how i interact with other guys (i just don’t cause i have no desire to...) i’m not angry about this particular situation but i just get so tired of these situations where i feel uncomfortable, explaining myself and having to so so much back and forth for him to even remotely understand why some things are just not ok to me. i know he doesn’t do it on purpose to disrespect me but sometimes i just can’t help but feel a slap to the face. idk at this point am i overreacting on this one