Thread.
Firstly, the definition of sex is not vague. It's based on primary sex organs (gonads).
If I told you that a nilblatt is "anything that identifies as a nilblatt" would that satisfy your curiosity? No.
It is not transphobic to point out this fact.
Identities are cryptic brain states and cannot serve as the basis for physical attraction.
If you see these being used in your child's classroom, you need to speak up and object.