This is my point exactly — whether someone is monogamous or not, it’s a poisonous mentality — but it’s also the prevailing one. “You looked at some other woman!” “You’re being awfully friendly to MY wife!” Everyone is so damn territorial! You can still be monogamous and not think and act that way, but it’s pretty rare, because there is such a huge amount of conditioning that this is the way you are supposed to behave. It’s seen as a badge of commitment and caring, even though all it really is is insecurity and possessiveness.
If people truly cared about their partners well-being, that could potential extend to them caring about or spending time with other people. That’s what is known in the poly world as compersion. It means your partner’s fulfillment and happiness makes you happy, even if you are not always a part of what creates that state of being. But that is not the world of mainstream culture at all.