"Women aren't interested in programming, so none of them go into programming professionally.
Why don't women go into programming professionally? Because they aren't interested."
Separate out whether women are interested from whether women are being forced out for other reasons (harassment, gender imbalance in the field, etc).
IF women aren't interested in programming, THEN none of them go into programming professionally.
IF women don't go into programming professionally, THEN they aren't interested."
I have 2 better questions to ask:
1) If fewer women than "the ideal" are interested in STEM, then why is that?
2) Why do we think a perfect 50/50 ratio across all fields is "the ideal," when "nature" might not deem it so? (note: this is NOT an "argument to nature")
Positive (for males) gender prejudice in a wide array of higher-status, better-compensated jobs drawing men disproportionately elsewhere. Which is kind of a mix of both of your options.
Come on.