As long as the woman is clear about it then your guy friends chasing that which cannot be caught is on them. Love is not a transaction and even if it was women have every right not to do business with people they don’t want to do business with.
In your example if your SO wanted an open relationship and you didn’t then that is when you say “if this is a deal breaker for both of us then we had a good run but I’m sorry.” It sucks, sure, but if you turn around and tell her that she can’t do what feels right to her then all you’re doing is using the relationship to pressure her into monogamy. It doesn’t fucking matter how popular or rare an idea is because you and your SO are individuals and have your own, individual needs. I mean christ, in your example you’re mad because a minority of women are supportive of her choice and that somehow makes it wrong? Do you hear yourself?