I would assume he is talking about genotype. Which is either male or female (except for the rare genetic abnormalities) and cannot be changed.How are you defining "biological"? What is the end goal here? Is it more useful to track what someone popped out of the womb with, or how they actually function in society?
As to the original question, I think there should always be a "dont care to say" type of option. Or just ignore the question.