Strongly disagree with previous self here. I do not think replacing "gender" with "sex" avoids complaints or "philosophizing", and "philosophizing" in context feels like a shorthand/epithet for "making this more complex than prevailing, mainstream views on gender."

For a start, it seems like even "sex" in the sense used here is getting at a mainly-social phenomenon: that of sex assigned at birth. This is a judgement call by the doctors and parents. The biological correlates used to make that decision are just weighed in aggregate; some people are always going to throw an exception. If you're not asking about the size of gametes and their delivery mechanism, the hormonal makeup of the person, their reproductive anatomy where applicable, or their secondary sexual characteristics, then "sex" is really just asking the "gender" question but hazily referring to biological characteristics instead.

Ultimately, gender is what you're really asking for. Using "sex" as a synonym blurs the data into unintelligibility for some LWers; pragmatically, it also amounts to a tacit "screw you" to trans people. I suggest biting the bullet and dealing with the complexity involved in asking that question -- in many situations people collecting that demographic info don't actually need it, but it seems like useful information for LessWrong.

A suggested approach:

Two optional questions with something like the following phrasing:

Optional: Gender (pick what best describe how you identify):

-Male
-Female
-Genderqueer, genderfluid, other
-None, neutrois, agender
-Prefer not to say

Optional: Sex assigned at birth:
-Male
-Female
-Intersex
-Prefer not to say