They can't be forced out if they didn't start in the first place (because of lack of interest). When I was at University, there were way more guys studying engineering and CS. Were they forced out before they started?

There is literally nothing circular about that. You're just saying the same thing twice.

Yes, there is. Allow me to rephrase it using Boolean logic to elucidate:

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")

Why are there less male elementary school teachers? Are they being forced out due to gender prejudice or are they not interested?

> Are they being forced out due to gender prejudice or are they not interested?

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.

On the other hand, saying a gender imbalance in the field is causing a gensee imbalance in the field....

Come on.