"Why can't you see you are just a pleasant diversion for them so they don't have to deal with the real issues in their marriage?" or any variation of the "Duh, you're being used, moron".
My response: Their real issue seems to be that they want each other to be happy.
"Look, they have already made a commitment, the only one society recognizes, to each other and have a real family of their own. You will never be an equal member of that."
My response: Why should I want a three-way marriage when I don't want a two-way one?
"Are you proud of yourself, breaking up a family with kids?"
My response: This is about building a family, not breaking up one.
The most common:
"Poly is just a fancy excuse for cheating."
My response: If everybody knows and accepts the situation, where does the cheating come from then?
Me: bi female in my twenties