People keep saying "it's his first serious relationship" like they've been dating for 2 months or something, but they're literally living together at this point. When you get to that stage of a relationship, your daily life revolves around your partner; you go to bed together, you wake up together, you coordinate your daily meals together. You're so deeply involved that you shouldn't just be able to vacate them from your mind in a moment of hardship to think "man i should go have a one-night stand." That's why it seems so messed up, at least to me, that Kaji needed to "come to his senses" at all.
Like if this happened in chapter 40? Sure, but at chapter 80? At this point in their relationship journey? No.
oh, come on! fukami is kaji's first serious relationship and we know he used to be a player, so it's normal (and very on character) that he thinks about going to a gay sauna, cause that's what he (most likely) used to do in the past
but not anymore! and he is trying!
is it cheating? yeah maybe?? in a way??, but it's not like he is ACTUALLY going to DO IT!