I’ve seen three types of answer to this question:
1) A circular definition e.g. “a boy is a person who identifies as a boy” or “a boy is a person who uses masculine pronouns” or “a boy is a person who is generally perceived by society to be a boy”.
What are they identifying with?
Pronouns are supposed to describe reality, not define it. What is the material difference between a person who uses masculine pronouns and a person who uses feminine pronouns? In French a table has feminine pronouns. If it had masculine pronouns it would still be a four legged piece of furniture with a flat top providing a level surface for working, eating or drinking at.
2) A word salad of a definition, perhaps including free spirits, mystical essences, limitless feelings or existing outside of societal constructs.
Word salad indeed. Also unclear how this relates to being a man or a woman. Either it is so deeply personal to the individual that it defies description, or it is generic enough to be used as a label for half the population, and can be described. Pick one or the other.
3) Attacking the question, e.g. “how regressive of you to define boys by their genitals”, or “it’s none of your business what’s in someone else’s pants, you pervert”.
In other words, going on the defensive in order to detract from the fact that it is far more regressive to define little boys as being made of slugs and snails and puppy dogs' tails, as opposed to little girls who are made of sugar and spice and all things nice.