Where exactly do people get this idea that fandom is dominated by straight women from?
No really, source please. Otherwise it’s just a baseless assumption probably heavily influenced by society’s heteronormativity.
Back in the 1970s and 1980s, that was how media fandom presented our sexuality to both ourselves and the outside world - it was a mirror or reflection of the fact that many gay and bi fans were closeted. It was not until the 1990s that more and more fans felt comfortable enough to start disclosing that they were not - in fact - heterosexual. This too was a reflection of social trends. Today, some studies show media fandom to self-identify around 35% heterosexual and the rest is a combination of gay, bi and asexual: https://www.academia.edu/8805593/A_Notational_Erotics_Locating_Queer_Female_Desire_in_Supernatural_Slash_Fanfiction I am certain the numbers vary from fandom to fandom and from country to country. Now why we still cling to the “facts” of 30+ years ago is, as you pointed out, a whole different set of problems.