IMHO this apparent gender gap has less to with why there are so few female programmers than it does with asking why there are so many male programmers. If one looks at the type of men that are in this field the answer is obvious but not one the male ego wants to accept.

Examine the social role types in the male programming profession and simply note the types that are missing. How many 'macho' programmers are there? How many mechanically inclined males are there? How many Adonis's?

Females who lack these qualities have many socially acceptable options and avenues to control the way they choose to use their personalities and talents than males. Men without the above listed personas can achieve self-esteem, ego, and financial stability in this business, thus it only makes sense that this field is a magnet for males with intellectual talent and are competitively/socially gender challenged.