Probably many of those things, especially the not having enough in common, but I'm guessing we're primarily looking at garden-variety conformity here... when I find myself in a group of people who are strikingly different from myself, even if I'm made welcome, I feel silly. When I realized there aren't many other married women on LessWrong, my immediate reaction was literally, "Why aren't there others? Am I doing something stupid? Am I a bad wife? Should I be doing housework right now?" This all happened within seconds. I was able to recognize these thoughts as maybe not totally rational only because the housework thing sounded silly, and I still did some vacuuming.