I think you're onto something with the idea of female characters not being written with agency. People often say "but the female characters are written badly!" and this may be true, but badly written male characters still get plenty of fic. The difference, though, is that a badly written male character still usually has agency - he will have a goal, a personality and a reason for existing in the plot. So many female characters, though, are there to be reacted to by men (viewers or characters). And yes, definitely the lower numbers of women in most canons are an issue. While male slash fandoms spring up around anything with two or more male characters who spend time together, some of the liveliest femslash fandoms seem to have either *just* two major female characters who have a strong onscreen relationship of some kind (e.g. Legend of the Seeker, The Devil Wears Prada) or an enormous cast gathered over many years (Harry Potter, DCU and Marvel comics, soap operas). An exception to this rule might be The Good Wife - while most of the fic fits the first category (Alicia/Kalinda have a strong onscreen relationship) there's a lot of other female characters around, mostly written with agency.

Last night I was watching a crappy movie (I won't say what it is as not to spoil people) and this post made me think of the femslash options. For male slash, there's four attractive, fit men who spend a lot of time together. For femslash, there's one attractive, fit woman...and two female ghosts. One has no agency and the other is a child. While it does pass the Bechdel test (the living woman and the girl ghost talk about various things) there isn't a femslash opportunity; the opposite is true for the men.