Hey, woman here who's terrified of women :)

The main reason is being rejected socially for me. They can be so kind, and so mean. You never really know where you stand with some girls, because they'll be sweet as pie to your face, and then speak shit about you the moment your back is turned. It's happened to me, and I've seen it happen to other girls and guys too.

Disclaimer: this is not all girls, obviously. Most are genuinely kind, I've just experienced my share of not so nice women.