NTR knight's story is the only one i can think of which involves controlling a male MC who is oblivious to his partner(s) being seduced. On the other hand, there are a ton of fem-protag games where you have sex with other men behind your unsuspecting partner's back. These include; Minako, Yukino, Young Wife Souffle (you can't find it on this website but its fuckin good), mischief war, tamami's hentai adventures, the fall of juliet (kinda), leanna's slice of life, Ane Kasegi, and some other ones.