(Agreement or disagreement on open relationships being immaterial to open-mindedness. The open-minded attitude is to consider what might make a stance true. Instead, to preserve your priors on open relationships, you're forced to assume a bunch of things, such that I am lying, and that I am unhappy, etc...)
Your comment about the social zeitgiest is interesting. Where I live (in western Europe) open relationships are quite unpopular (despite being more popular than they've ever been). I actually expect the same to be true in the US, outside of some affluent circles (if we're talking about in-the-flesh relationships).
How would you define conservatism?
(I don't consider my definition as negative btw. I have a conservative mindset on quite a few things where I think improvement is hard, or we're stuck in a local optimum and the cost of getting out would be currently unbearable.)