The way that somebody feels is a real response to emotional stress.
If you love somebody, shouldn't you be taking their feelings into consideration?
I've witnessed some polyamorous circles dismiss or marginalize feelings by creating a landscape of non-accountability, wherein the culture agrees to a standard for divulging sensitive information about their relationships. They all agree upon the terms of that communication. Then, to that culture, those conditions become an ethical baseline that contributes to a narrative that looks like this:
"Well, based on our agreements, I've done my obligation by telling you that I'm going to be gone tomorrow night. I will have sex with that person."
Whether or not setting a baseline of "moral principles" surrounding communication is ethical can probably be debated - I don't believe treating others like crap is "moral" in any sense of the word, even if it's under the guise of agreed-to expectations - but within this narrative we see two things:
1. A persons' feelings are being summarily dismissed;
2. Accountability for contributing to the feelings of another is being avoided.
In effect, the practice of ethical communication becomes a license to do as they please without fear of accountability or being told no. In effect, the narrative is allowed to continue:
"Your feelings are your own. You're just going to have to deal with it."
In my mind, this is unacceptable behavior. It's not treating the other party in a way that'd be loving or respectful. Nobody should be treated this way. This is a practice of control. It's a practice to get what you want by deflecting feelings behind a shield of self-righteousness - perhaps brought on by a delusion of intellectual or moral superiority.
Get down to basics. Look at a person. Look into their eyes. If they're hurting, and if their feelings matter - and in loving, committed relationships, they should - it's time to embrace your responsibility for her frustration and help change it, rather than dismiss it.