This has to be one of the more extreme instances I've observed of redefining reality to fit your own ideological purposes. Presumably the sex ed teacher is following the ideal of liberal autonomy in which we are supposed to be self-determined in all things which matter. If our sexuality is fixed it can't be self-determined, so it helps the theory if our sexuality can be made fluid.
This is the basic idea behind queer theory, which has been promoted widely in schools in the US:
For the queer theorist, all unambiguous and permanent notions of a natural sexual or gender identity are coercive impositions on our individual autonomy - our freedom to reinvent our sexual selves whenever we like. Sexuality is androgynous, fluid, polymorphous ...
So we shouldn't be surprised if activists aim to overturn heterosexual norms. I hadn't, though, expected the bold move of relegating heterosexuals to minority status.