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!