I have read through the majority of this thread and honestly... why does it even have to be "labeled?" It is their life choices, done in their bedrooms. If you are asked to participate then you have a voice in what is done, if not, it's none of your business. If they make it your business then you can either respond or ignore it. Personally I see my relationships in that area to special to "share" a lot of details openly. It's no ones business but mine, period.
As to the opening statement about most sub missives being bi. Ummm... i'm sub but don't consider myself bi, even though I am currently serving 2 Dommes. Heck, my actual sexual experience with women could pretty well fit on the head of a pin. It isn't the gender that is the turn on for me, it is the power exchange and how well their dominance fits with my submissiveness. Their are as many men out there who don't turn me on either... because the dominate connection isn't there.
So, I don't consider myself bi, hetro, bi curious, or any other "label" i consider my self submissive. Period... a submissive who was damn lucky enough to find the dominate(s) to fullfil that portion of me.