I wouldn't sweat the whole kink-community-not-getting-it thing. For a group of people who should, logically, be super open-minded, I've not been particularly impressed with most of what I've seen in this regard.
As for women who suddenly change their tune when your boyfriend brings it up, I think there's two possibilities for why it happens:
First, that they think it's abusive or exploitative in some way. People, both men and women, tend to start out with more sympathy for women by default. Not sure how much is a learned behavior and how much is innate. But people will often react to a situation that seems one sided against a woman in a way that they would not react when the genders are reversed.
It's a double standard that treats men callously and treats women like they lack agency, but it happens.
The second is a little more self-involved. There are some women who have their ego wrapped up in their sex lives. It's not that they don't enjoy sex, but there's more to the circumstances than just whether they like and are attracted to a potential partner. The idea that they can 'take' a man from another woman, even if just for a night, is important to them. This is why they are okay with, or even specifically attracted to attached men.
So when this man who has a girlfriend tells her that the girlfriend knows and is even enthusiastic about it, it loses the allure. She isn't making him stray from his woman with her sex appeal. In fact, there's not a woman who can, because seriously - what man would ever leave a woman who wants him to fuck other women?
It's taking away power that she thought she had, which is what this was about for her to begin with.