open relationship doesn’t excuse the situation and the op is right to feel this way. you just don’t trust ppl like that to suddenly become committed to one person after sleeping with the whole town while the one person they love was around them all the time. it’s like taking his presence for granted while playing around, and now he wants to be exclusive? nah, no way i’d believe that. and imo it’s true, his character feels like an easy cheater
And a side-note: it's only an "open relationship" when both parties have mutually accepted that fact, otherwise it's just cheating. Currently the characters are not yet a couple, so they are free to fuck around as they please. The monogamous one can still feel hurt by the other one's sexcapades, which is sometimes enough to kill budding feelings.
Bro even has a group chat he's a playboy through and through. Ik at the end, he's gonna end up together with the uke but dudeeee, i just CANNOT trust the seme to be loyal. He seems like the kind of guy to sleep with other people when his lover isn't around. Author, please prove me wrong and make this guy do a 180 at the end of this manga.