No I do not find it morally right for an adult to have sex with a teenager under their control. But that is the difference in our logic, you assume that the teacher has manipulated and coerced the student into banging her. Where as I think that, though that certainly has happened before, that is probably not the case as the dude is 18 and probably fairly good at making his own decisions. Also using the word teenager in your statement makes it more inflammatory, but obviously there is more to it than a "teenager". If he had robbed somebody the news headline would read "18-year old man".
And I don't see why you find it so impossible for these two to simply have slowly grown feelings for one another until their educational relationship became ignorable. She is decently attractive, I assume he is decently attractive, they are both sexually developed; that is all there has to be for mutual human sexual attraction, regardless of professional or whatever relationship. Once again I understand the scenario you have presented and realize that it can totally happen in a student/teacher relationship, but I just don't see that as the case here.