I get your point, and it's a good one.

You didn't even mention the part that of all the ways you're likely to know about your sexual dimorphism, what the medical staff (if any) wrote down when you were born is least likely to be accessible to you. Sure, official documents propagate things that are recorded at birth, but you don't learn them directly.

On the other hand, there's an argument that everyone knows perfectly well what the questions means.

Sex? Gender? Assigned sex at birth? Presumed content of sex chromosomes? Mars or Venus?

So it also makes perfect sense to me to just answer the question. Some battles not only aren't worth fighting, they're not even battles.

(Unless you give feedback. "Dear survey writer: I object to your use of the term 'assigned sex at birth' to gather information about physical sex. I know what my sex is and was, and it is not ambiguous. The implication that I ought to defer to a mistake in identifying my physical sex, if one had happened at the time I was born, is rather insulting. Please find a different term that everyone can agree is sufficiently respectful." Then there's actually an engagement.)