Because i only identify as part of a group based on hobbies or recreation, not based on something that is a component of my personality or internal workings.
I can sum it up this way, i've said this before - i go to nude beaches to be nude, not to be a nudist. I joined roller derby to skate, not to be a roller-girl. I have relationships with people because i like something about the person, not to be polyamorous.
Also, i am not out there looking for people to become involved with. I don't think of myself as "available" even though i do meet some people's criteria for that definition.