I think going the route of asking what gender means etc might be the most helpful. Even if I can't win the argument (given how moveable the goal posts are), it might at least make some people stop and think a bit.
Is a good one, because it makes people think.
You might also slip in the word lady brain.
Because, essentially, that's what they're saying. That men and women think differently. Not because of their bodies, not because of the way they are treated, just an innate, different way of thinking. And, in the case of trans, men are thinking like women.
If you could steer it in that direction, you will have people definitely questioning it. Most people will definitely pause if they are being told that women think in a certain way. Because they know damn well that it will be a way that disadvantages women. Irrational, emotional, etc.
And, it's clearly nonsense. Men and women do not think differently.
But it's quite hard to dismiss thousands of years of assumption unless you force the questions.
'Lady brain', to anyone with an ounce of critical thought will certainly start the ball rolling.
"I don't mean a lady brain! I mean an inner feeling"
So ask them describe it. With every single word in the English language at their disposal, describe that feeling.
And watch the sexism unfold...