In many states it is a crime for a teacher to have sex with a student, even if the student is 18. Considering that teachers have unique access to students and are expected to act in loco parentis, in a parental role, many people consider this particular abuse of a teaching position to be an action that merits prosecution as a felony. As a teacher, I agree with those who want it to be illegal and not just grounds for firing.
You're going to send your daughter to school for close to 200 days a year, where her future depends on what we teach her and even what we write about her in letters of recommendation, and your tax dollars are going to pay for us to perform those duties whether you like it or not. Is it too much to ask that all teachers keep their hands to themselves when they teach?
Not at all. It’s clearly an ethical violation and apparently in this case a legal one.