Their biological sex might be indeterminate but there's a good probability that their gender is male or female.

If a TSA officer can't determine someone's gender from appearance, they ask "What gender are you?" or equivalent.

I was asked this once. I didn't give her a clear answer (not intentionally...just got thrown by the question). At that point she asked "Do you want to be patted down by a male or female officer?" I said female. She prodded my leg a bit. I was good to go.