It's a conversation that they had; Hanaan having a lifespan of around 1000 years means that she was bound to run into at least one person Kismut had a relationship with. While being monogamous she has no problem with Travis seeing other people as long as they communicate and it's not just him sleeping around without saying anything. (Sidenote, but there's a reason I call his lovers the "Group Chat"; they are all in an actual group chat and know about each other.)