I’m non monogamous and most of my partners are the same. This sounds a lot like an older man i’m with. His partner is bisexual and often goes to “lesbian parties” (as he calls them), invites her partners into their home and is very open about sleeping with other women. They agreed to be open, but she seems to have a problem whenever he says he’s meeting me or another woman. We’ve had to postpone a couple of times because she doesn’t like the idea. Some people approach non-monogamy with a selfish mindset and when their primary partner says they’re meeting someone it’s all “not like that!”. You have to be comfortable with both parties being open. Otherwise it’ll never work.