I'm with you on not understanding this brand of open-mindedness. I am ok with gay marriage and polygamy, and also with monogamy and singlehood (or whatever you call it). I think people should have the right to choose their life path - isn't that the intent of "pursuit of happiness"? If some woman herself feels that she is wasting her degree by being a SAHM, then she should do what seems right for her, but that doesn't mean she gets to choose what seems right for other people. That's just as unfair as women being paid less than men for equal work.
When we come to the Christian bit, I think I might start to understand where people are coming from with these attitudes, just maybe. I'm not Christian, although I was at one time. I'm pretty firmly agnostic. I have felt judged by Christians fairly strongly and multiple times in my life. I think what happens (with this, and perhaps with other issues you mention) is that people are expecting to be judged, so they jump right in with a preemptive first strike. Of course, it's not fair to you that they do that. I just want to make it clear, I've been NOT judged for my beliefs by way more Christians that I've been judged by, and I try to take people on face value. I can't resist saying this, because I think it's funny (and because it's true), some of my best friends are Christians. But from my experiences feeling judged by Christians, and thereafter having to intentionally
choose not to prejudge the next obvious Christian I meet, I think it must be a preemptive first strike thing.
Probably that's true for a lot of the issues you brought up - people in polygamous relationships are likely feeling a little bit beleaguered by popular opinion, like they are outnumbered and judged by us monogamists. So they feel like they should dismiss your opinion before you can hurt them by judging them. Again, not fair, but perhaps at least understandable.
And maybe it just has something to do with our love for underdogs. To use a bit of an odd example, when my two goats are fighting I feel bad for the smaller one who isn't as good of a fighter. When people see a lion take down a gazelle, they feel bad for the gazelle - when the gazelle gets away they feel relieved, not bad for the lion. I don't know why we have this tendency, but I've observed it in myself and others over and over again. So if the perceived cultural majority is monogamous, people feel the need to champion the polygamous. If the prevailing cultural wind is (or in this case has been for hundreds of years) Christian, people feel the need to champion the rights of the . . . I dunno, atheists, I guess? It fits with our apparent nature, but it must be lonely to be truly open-minded and in the majority. I try to be truly open-minded, but I really don't usually seem to find myself in the majority categories. Anyway, I just went into a big sociological discussion, when what you really were looking for was probably just this last bit.
I also end up feeling like I don't fit a lot of the time. I was too weird hippy to fit in properly in University (though I did just fine anyway), I was too academic to fit in with weird hippies (though I'm friends with many anyway), I'm too conformist to fit in with "non-conformists" who always just seem to be going against the grain just to be ornery, but too non-conformist to fit in with the herd, who seem to do things without questioning. So I totally sympathize. To be honest, I'm starting to think that that's kind of how everyone feels. I think maybe that's the big secret that can make it all a tiny bit easier - everyone feels like they don't fit in, like they say the wrong thing, like people think poorly of them sometimes. Don't sweat it. I think you sound nice. And like you need a hug.

And I think everyone has their own little area of intolerance . . . if I'm honest, I'm afraid of businessmen. In their crisply cut suits, with their briefcases, and their clean-shaven faces and their short hair . . . I just don't trust them. It's completely irrational and unfair, I know. I imagine they have much the same proportions of nice to mean, clever to dull, etc, etc, as any other group. In fact, I imagine many of you are waiting for me to admit that I'm joking, but I'm not. It's strange, it's unexplainable, and it's a little bit shameful, but I'm prejudiced . . . against businessmen.