Why do you hear sexual orientation or gender identity and immediately think about sexual behavior? No grade school teacher is discussing sexual behavior with children.
Well, other than the ones who are.
But to answer the question, it’s because I lack the imagination to figure out how one explains sexual orientation without explaining what that means. Sexual orientation, after all, is about with what gender one prefer to have sexual relations.
And short of a groomer hoping to persuade a kindergartener to indugle in sexual relations, I also lack the imagination to dream up an excuse for discussing sexual relations with a little child.