That's partly the issue of concern, isn't it? Giving children the idea that if they don't fulfil a traditional male or female role, then they must be some alternative gender identity.
Many people don't agree with this stance, both from a conceptual point of view, and that in practice it may lead impressionable children to want to modify their bodies to fit some idea of gender identity they've had instilled.
I think it would have been more enlightening if they'd separated out the question, to see how many people would be okay with sexual orientation being taught, but not gender identity. Or vice versa.
They're two very different, and somewhat conflicting, concepts. For example, is homosexuality an attraction between people of the same sex, or the same gender identity? This is a contentious issue even amongst adults, never mind teaching it to children.
Many people don't agree with this stance, both from a conceptual point of view, and that in practice it may lead impressionable children to want to modify their bodies to fit some idea of gender identity they've had instilled.