1) "Geek" culture is not heterodox. It's very accepting of fat guys who like faux-Mexican food, and anime; but not so accepting of other non-conformists. Many women are not fat guys who like faux-Mexican food, and anime.
2) Women are encouraged to work hard. Men are encouraged to be talented. This is true of teachers, parents, peers, and managers. Coding is something that requires more talent than hard work (at least, to get started); due to the high barrier of entry.
3) There is some ugly sexism in parts of tech. There is ugly sexism in other career paths (finance, sales); but we can do a lot better.
4) Some women choose to have more personal time, rather than staying up till 3am coding. It's hard to tell if this is really relevant - Patrick McKenzie claims to have run business on 5 hours a week. And no-one here (I hope) think driving code monkeys on 80 hour/week death marches is really a good idea (due to tiredness-induced bugs).