You know, I thought that at firs too. I was going to say that it doesn't make sense to say a bisexual has B=0.5 and that we should have B_male and B_female. However, as I tried to think of a situation where you'd want to split that into two dimensions, I couldn't come up with one. As long as we read B as a relative preference it makes sense to say bisexuals have B = 0.5.
