So, if instead of being a teacher/coach, let's say he was a 35-year-old "assistant manager" at a convenience store and had a relationship with an 18-year-old customer. Would he belong in the registry? If she were an employee instead of a customer, would your answer change?
A 35 year old man, a teacher and a coach, should be able to keep his hands off his students. If consensual, they could've waited until after her graduation to consummate their relationship. It wouldn't be the first time. There are many high school teachers married to former students. Is this relationship heading toward marriage?
If the victim had been a confused boy, and the teacher/coach were a gay man, would you feel the same?
Parents shouldn't have to worry about their daughters or sons being seduced or pressured into sexual relationships with teachers or coaches. For every consensual relationship we might hear about, there are numerous less consensual relationships involving the abuse of authority.
And your example of an assistant store manager and a customer is a bad example. You would've been better off making the 18 year old high school student an employee.