I am wondering what you mean by "crush" if it is turning out to be painful. I have always enjoyed developing crushes on people, when there is no expectation or hope of it becoming a relationship. For me I guess that feels like a friendship with a little extra layer of excitement that makes the time spent together feel especially good. I don't think this is harmful to anyone.
I don't know what happened in your case but it sounds like there was something else going on. If your pain is from frustration or disappointment, does this mean you had hopes of a (beyond friendship) relationship developing, or the crush being reciprocated?
The problem I see with people "staying away from" monogamous people is that it sets monogamy up to be very lonely. My mother seems to believe I shouldn't befriend men, since I am married (she doesn't know the half of it) but I see where that expectation of marriage got her -she's twice divorced and has given up dating. I would think even the most deeply monogamous deserve friendships and interactions outside their marriage.
Married to a monogamous man 15 yrs, mother of 2, dating C 3 yrs, and in a romantic friendship with L more than 20 yrs