Graves is married. Throughout the events in the movie, he actually has a wife except he lives in the city and his wife is down in the country. Their marriage is probably a mutually beneficial arrangement although neither likes the other much, but at least they're polite to each other if distant. Hence her not knowing that Grindelwald is impersonating her husband.

Enter Newt. Newt finds Graves and over the course of Graves' recovery, they fall in love and start a relationship. Newt can be conflicted about their affair or completely nonchalant or somewhere in between. I just want something in this kind of setting, that is, Newt being the other woman. I don't mind if it's super angsty or with a happy ending or what have you :)