It’s really more of an issue of abuse of power. Do you really want school teachers of either sex having sexual relationships with students? Seriously?
The abuse of power argument is more compelling, I agree with that. I don’t see this case as a matter of child abuse.
That being said, it is not uncommon for college and graduate school teachers have romantic relationships with students. The abuse of power dynamic is present there, too, but it’s not really all that scandalous.