I was a girly girl as a child, but I still wanted books about girls. When I couldn't get them, I made up my own, from about the age of five.

Me too! I was firmly convinced that Merry in The Lord of the Rings was a girl, and so disappointed when I realized he wasn’t. I loved the books though, so I spent a lot of time making up female members of the fellowship.

I've even wondered if that's some of the reasoning behind slash,

I think that is part of it. When something you love only gives the male character depth, well, then it isn’t so strange those are the ones interesting to write about. But I think it’s also a matter if you enjoy imagining yourself as part of a ship, or to watch it. I like to identify with characters I like, and find that difficult with male characters. And though I know a lot of women thinks it’s hot to imagine two attractive men together- I only find that boring. I can, and do, ship m/m on occasion, but I never leave the g-rated stuff because I’m interested in the dynamic, not the sex.

When I first got into fanfiction I found it hard to find female characters that were interesting, in fics, and not Mary Sue's, but I think that has changed a lot.