I mean, that doctors and lawyers were substantially less sexist compared to engineers in the 60's and 70's also strikes me as less than plausible, though I'll grant that'd be a closer competition than comparing them to engineers now. Do you have any evidence of an advantage there?
But the 60's and 70's are when the representation levels started really diverging. That's the key part.
That they may be less sexist now, after having achieved gender parity or something close to it, is hardly unexpected. Of course a field with roughly even numbers is usually going to be less sexist than one dominated by one gender or another.
You'll have to spell the argument out. My claim is that relative sexism in STEM puts women off now. This does not require me to commit to the claim that relative sexism was a dominant factor in women's career choices in the 60s. Note, however, that the issue is sexist attitudes in society as a whole, not just engineers being sexist. Even if 60s lawyers are just as sexist as 60s engineers, the idea of a female lawyer (especially a junior one) may still be more socially acceptable.
Asserting that a gender balanced field is less sexist than a gender imbalanced one isn't very interesting. The interesting part is that law and medicine used to be just as gender imbalanced, and then steadily became less so.
This is the crux of the issue that you're avoiding grappling with. Saying "well I don't care about the history" is irrelevant, it's still the most important part whether you personally care or not.
It's like looking at which countries are desirable to immigrate to without grappling with patterns of development, despite the obvious fact that the biggest thing that makes countries more desirable is being rich/developed.
