It is way too easy to look down on those that are different then you. I think that is part of human nature to a degree. But we can learn to overcome these prejudices. I grew up in southern USA and I saw a lot of racism. (I am a white male BTW.) Interracial relationships were very frowned upon. But when I talk to some people from back home, that is not really an issue anymore (because they have moved on to gay marriage). However, it is progress. Both the left and the right keep moving left over time. Many conservative parties of today may be seen as liberal by historical standards.
So for me, the short answer is that it is human nature from not being exposed in a human way to someone of that minority. For example, if we talked about furries (a kink in which people who take on an animal avatar and may dress up during sex) I am sure that on here, there would be some who would poke fun at them or pick on them to a degree. But if a member stood up and said they were a furry, suddenly people would get very polite and may actually listen to a personal story.
You can test the waters some by challenging bad assertions. For example, you can ask where the person got their data that women in open relationships were abused. I have heard a similar statistic to "explain" lesbians.
Another approach is just to some out and say that you are surprised by the hostility toards people who have a different relationship model. You may find out there is a silent majority of supporters. Or you may get a lot of curious questions. But if you did that, you may be the token open marriage people until someone else comes out.
I work around a lot of scientists. When the topic of gay marriage came up, there was one guy who was against it. His statement was, "If you allow gay marriage, then by the same concept, you have to eventually allow group marriage." That took several people by surprise because they saw the logic of it. But then they agreed and said, "I don't have a problem with that."
I see it a lot like gay relationships. They won rights by coming out and showing that they were just regular people. I think poly is in that beginning state of winning society's trust.