Clearly there are plenty of things we all (to a first approximation) agree that you cannot in earnest say at your workplace.
There are other ways to combat illogical ideas.
As an atheist I would not fire people for being believers (even if it were lawful to fire on that basis) --just because "I know better". If one of them tried proselytizing, by law I can't do anything about it, but even if I could, there would be better ways to combat irrational appeals and sowing of fear.
That said, what if the engineer has "only" presented the ideas as a private person at a "men's club"? If that's not okay, then this infringes upon people's speech outside of work as a citizen --that's chilling. There are Walmarts out there.
I believe in making tech an attractive career path for women --as a society we underutilize them and don't extract the value we could --as the soviets did from their women. But at the same time, I am sensitive to kerbing speech.
Instead, he had the poor judgement to publish in a workplace forum an argument that the women at Google tended to be there due to reduced standards for hiring women, because biologically women tended to be less suitable for the kind of work they were being hired to do.
Timothy Lister had a term for this kind of thing in _Peopleware_: he called it "teamicidal". If you do something teamicidal at your dev job, you should expect repercussions. You are paid to contribute to a team, not to fuck it up in an effort to remake it in your image.
(I think Google took the easy way out here, by the way, and that there were more productive ways they could have responded to this, rather than let it fester for days until they were cornered into a dramatic response. But: I don't work at Google, and my epistemological certainty about this is somewhat low.)
That said, I don't agree with sacking someone because they said something unproven and because it can make people feel bad. I know some Googlers, men and women, and they tell me stories about people getting fired over small indiscretions (a bar, for example) and about people getting called in an office for saying off-handedly "lady" or girl. While not getting the same reprimand for saying guy or boy.
On the other hand belittling "middle America" is kind of a pastime with many a Googler. (When you point out that South Florida is heavily Hispanic and that those Hispanics vote republican, they find it hard to believe, for example.)