This type of trope never made sense to me for BL stories because if the character thinks his partner is gay i.e. not attracted to women, why would he think his partner is cheating on him with a woman? It would make more sense to realize that the woman is probably a friend/relative and could be resolved with a simple question.
i definitely agree with you, most bl misunderstandings could be resolved so easy if they just communicated ╥﹏╥ however i also think in a lot of bls they never actually talk about their sexuality and often assume their partner would prefer a woman to them. ex: in a lot of royalty bls it’s often brought up that the heir must get married and the partner assumes they are just a play thing for the time being. again could be solved with a little communication lol
classic misunderstanding, thinks his sister is his lover trope