Originally Posted by YouAreHere
From my own introspection (not to speak for Matt, but I tend to not use the word, either), I think it's partly because of the way it sounds... "Metamour" implies a relationship with my partner's other partner by virtue of THEIR relationship. It has no bearing on what MY relationship to that person is, and if I don't feel a personal relationship with that person, I'm very hesitant to use words that imply such. It's the difference between calling a person "My sister-in-law" or "X's sister". One term does a better job of keeping a person (emotionally) at arm's length if you don't feel a connection (or if you feel animosity).
I don't think that was what happened in this case, but I know what you are talking about. "My father's wife" is not "my stepmother" because I never had that kind of relationship with her. I don't dislike her, in fact i like her better than my father, but I just never had a parent-child thing going on with her.