The theory I've heard is that it's a manifestation of sexual anxiety in girls. The fujoshi culture in Japan is the biggest example. As girls mature, they start to feel attracted to men, but some of them are nervous about actually pursuing relationships. They're unnerved by male aggression and are uncomfortable with being desired by a man. So they retreat into a fantasy world of gay relationships between idealized men with a mix of masculine and feminine characteristics. Slash fiction, yaoi, boys' love, etc.
The men in these fictional scenarios act nothing like real gay men and the descriptions of sex are laughably inaccurate but it allows girls to project sexual fantasies in a way that doesn't make them insecure. There are no women in these stories so they won't feel uncomfortable by projecting themself onto a character who is desired by a man and they don't feel jealous of a female character who is desired by a male character they're attracted to.
A lot of young women have similar motivations for identifying as trans. They're uncomfortable with the female social and sexual roles, so they wish to become men as an alternative. Their idealized manhood resembles real manhood as little as the yaoi boys represent real gay men, thus the parade of Aidens. The psychological factors underlying fujoshi and FtMs are just about identical so it's no surprise that the former is a pathway to the latter.
I love their horror audios - The Magnum Archives. I'm really disappointed.
