The one shitty ex of mine who cheated, we were fluid bonded. I had asked him on multiple occasions if he wanted to change from monogamous to something else, asked on multiple occasions if there were things he wanted to try, etc. Instead he lied, couldn't keep his dick in his pants, and fucked the most disgusting person I've ever met bareback.
By disgusting I mean I don't think this person ever brushed their teeth, their shower/tub were full of dirty dishes because their kitchen ran out of room, etc. Filthy. Sickingly so. The whole fucking each other behind my back just adds to them being scum.
If my ex had asked I would have given my blessings for having safe sex with other people, but he was generally a garbage human being *shrug*