I'm not sure if you've watched Queer As Folk before, but i read an article a few years ago on why such a large percentage of their audience is made up of straight females when the show is primarily about gay guys. It's kind of psychoanalytical and also a little stereotypical but nonetheless it did raise some valid points about the audience, which are demographicaly similar to this site, despite the different mediums (tv show, fiction).
"Why are all these Straight Chicks Watching Queer As Folk?" [article]
I think that because this site does contain mostly (if not all) stories about gay males, straight men and lesbians aren't as attracted to it. Especially since those stories have already become such a large majority, the only people joining now are those interested, ie; the straight females, bi females and gay males. Maybe if the site had started off with an equal amount of gay male and female stories, more straight males and lesbians would have joined closer to the inception of the site, thus we'd have more lesbian stories and the GA community would probably be a little more diverse than it already is.