If what I remember, solidarity is the union of peoples with common interests, correct? C: (Correct me if I'm wrong).
I believe solidarity is a human need. C: We want to work together and avoid conflict. The human body hates stress and if solidarity help eliminates that then it could be viewed as a psychological need. It's no different if you're gay, straight, African-American, Jewish, etc.
BTW this is coming from a bi female since you wanted to hear answers from us. C: