Like mentioned early in this thread, make sure to keep distance between the MC and the women that would be sleeping with others. By not establishing a strong emotional connection and limited communication, the player could be rather oblivious to the detailed actions the women are engaged in unless they choose otherwise. Meanwhile the genuine romantic interests could be kept on a loyal path with MC, since they're more fleshed out characters, the player is unlikely to even care about the women with one dimensional personalities doing stuff in the background.