Student teacher relationship is certainly objectionable if it is there between a major and a minor.
But what about it, taking place between a major and another?
What does it mean being a major?
Doesn't it mean that you are matured enough to take the decisions of your life with a responsible independence on your own of your own?
In that case if an adult student decides to have sexual relations with her/his teacher, what is so wrong with it?
Will it disturb the equation of learning and teaching between the two?
I don't think it will, since such an equation depends not on their relationships but on the curiosity level of the personalities that both of them have.
Real strange people are we! We are not bothered about all the psychologically distorted sex between lesbians and gays, and are even making legislations for them to get officially married for that matter; but when it comes to straight sex between two consenting majors, our morality gets hurt!
Are we sane?