The trouble is that, whilst it's easy to determine whether or not the person stood next to you is shorter than you, competence is something far harder to assess. If your opinion is 'women are, on average, less competant', it would seem more likely than not that your default position would assume the woman stood next to you is less competent.

Not quite - if it's not a random person standing next to me, but someone also working in the area, I'd assume that the filtering has eliminated people less competent (if not, then we'd already be at 50/50 equality). To be concrete, I'd assume that any engineer at Google to be about the same competence as their peers. Though it sounds like if you're "diverse" you might have an easier time of getting in, similar to medical school? (Honest question.)

At any rate, how do we get to the facts? Saying it must be 50% by fiat doesn't seem like a way to truth. So far I see one side proposing there might be biological reasons and the other side being offended anyone could even suggest the possibility.

It would also seem that if we actively prohibit this line of enquiry we will never actually learn to accurately measure "competence" or "empathy", let alone enable humans to recognize those traits in others.

Measurement is key to understanding!

About the only "lie" in this debate (in so far as I can tell) is that we are all the same!

Sure - at some very high level of abstraction it may be "true". But as engineers, surely we understand that failure stems from false assumptions?

This does not seem as much of a trouble as having a preferential hiring policy for some classes of people and then having the ability of these people doubted not just at the hiring time but through their entire career.

To wit: I am a foreigner living and working as a software engineer in the USA. My English is far from perfect and a lot of people assume I am dumb just because of my thick accent. I failed few interviews because of this. I have absolutely no problem with that and am happy with my career. My achievements speak for me. I would not be as happy if this had been made a "diversity" issue and people with heavy accent were given advantages in hiring. This would forever invalidate my achievements as a proof of my competence. I would actively oppose any moves to establish such policies and would not approach any company that had them in place.