Its because it all extends out from the gays and their community, wherein a person is defined by their "top or bottom" status to a point where its basically used as a standin for being the man or women in the relationship. Even their subgenres of gays are defined along lines of how manly/womanly it makes you (with bears and twinks being the hard masculine or feminine appearances).

At least there, because its all dicks, they were forced to attach the status to other non-primary sources of sexual demonstration. But in the absence of that, you can just narrow it down.