That's the law in Arkansas (§ 5-14-125(a)(6)(B)), and a proposed law in Louisiana. (Right now, Louisiana makes it a crime to have sex with under-19-year-old students, but the bill, which unanimously passed both chambers of the legislature, would change that to sex with under-21-year-olds. The crime is a misdemeanor for the first offense in Louisiana, and a felony in Arkansas.) This might well be unprofessional conduct that would justify firing the teacher, but sending the teacher to prison?

The "abuse of a position of power" rationale doesn't work here, I think; the teacher has no more power over the student than the typical employer, or police officer, or many other people. Yet we deal with the extraction of sexual favors through threats in those contexts by requiring proof of abuse (whether in a civil case or a criminal extortion case), not by just presuming it.

Nor does the psychotherapist analogy work. To my knowledge, psychotherapist-patient sex isn't a felony, though it may lead to professional sanctions, and in any event the teacher-student relationship strikes me as quite different from the psychotherapist-patient relationship.