i actually haven't read many where the male lead already has a lover and mc swept in to steal him, i mean that gonna be like a honewrecker story. it's risque lol
but if you mean stories where MC thought ML was destinied with another (but not official yet) yes there are so many, especially in transmigration/Isekai story
oh i meant ao3 fics of tojou x mob LOL last time i checked there were a few! i think almost all of them happen after ayato and tojou break up for whatever reason in the fic
but same, i dont rlly like stories where mc is in an established relationship and they get taken away, infidelity is fr not for me

Lowkey I ship mob with toujo lol