I don't think it was a "nasty" reaction, since when he'd asked if they'd dated (also thus implying "have you had a sexual relationship/encounter with them"?) he was told "no" only to find out later that the person he'd thought was just a friend had had sex with his bf/prospective bf. Maybe that doesn't matter to you, but it does to many people because if it happened once, then it's easier for it to happen again (ie: getting back with an ex). It's not that having hookups is the problem, or having dated (because the uke dated in the past) it was the fact that that aspect of the relationship was *hidden* from him when he fairly explicitly asked about it.
I like how somewhat realistic it was, not super fluffy and uke didn't completely fall in love in like the 20th chapter.
what I didn't like was how the uke treated the seme and blue hair dude relationship, they weren't even boyfriends they just hooked up once and didn't kiss as much....it was nasty coming from someone who had multiple relationships and slept with other people
I could see how he would feel self-conscious about his boyfriend still being friends with his ex-hookup but still having that reaction was too childish.