Cases of “the other man” are typically like the situation you’ve described: the man is rarely at fault for the adultery because he doesn’t know or misunderstands the woman’s situation. (Don’t confuse this with the cheating boyfriend, though)
Why this is bad: It’s not necessarily bad. It’s only bad in the sense that when you look at how men are viewed when in this situation and how women are viewed, there’s a stark difference. Lori and Shane from The Walking Dead are perfect examples. Lori, believing her husband Rick to be dead, begins sleeping with Shane, her husband’s best friend. While Lori was convinced Rick was dead, Shane was only pretty sure – because he left Rick to die seeing no way to save him. However, fans reacted violently against Lori while finding dozens of ways to excuse Shane’s behavior – even when Shane continued to try to go after Lori, who refused him upon finding her husband was alive. So while “the other man who doesn’t know he’s the other man” isn’t necessarily a problematic trope, it does show the huge differences in the way audiences respond to male characters and female characters; the woman is always at fault (even when it’s not her fault).
How you can fix it: Since there’s not really anything wrong with this trope per se, there’s not much you can do to fix it. In this kind of situation, the woman is usually made out to be a selfish harpy using the unwitting man for her own personal gain or an abuse victim searching for love. Both of those tropes are highly overused and create the devil/angel dichotomy that you may want to avoid for the sake of keeping your characters complex and less predictable.
Bottom Line: The most you can do is make sure the woman who knows she’s cheating isn’t a lustful demon or a virtuous angel; make her complex, and you’ll at least put more creativity into this trope than most people.