I see videos of Crowder types asking randos on the street to do it, and of course, they always stutter and stumble their way around what it is (if they're well intentioned - of course lots just say the usual man = penis and woman = vagina answer.)
Most of the people that get caught out are just well intentioned people whose philosophy on questions like this is 'who the fuck cares' (which is valid).
However, I'd rather not be caught out. Usually I try to immediately split it into sex Vs gender by just asking them to clarify - "what is a woman according to sex or what is a woman according to gender" and then I try to go from there.
What's a good line to trot out in answer to stuff like this? It would be nice if it could become almost a set phrase answer so it could catch on easily.
Options:
"Why?" Again, put them on the defensive.
"How will you implement that definition in practice? How will you determine who is a woman when it comes to access to space?" Address the practical horrors of the things they advocate for. It's either invasive genital tests and/or an intense system of government surveillance and documentation.