It doesn't matter what the occupation is, it matters if the person wields some amount of influence and power over the other. If you are a boss you can't force yourself on your subordinates as they might fear for their jobs if they don't comply. In the work place there are procedures that can be followed so that two consenting adults can have a relationship, but even then, most companies will not allow senior to subordinate intimate relationships, even when the subordinate signs the right paperwork as there is still to much risk. The teacher and student relationship will always be senior/subordinate so there isn't any flexibility to allow a consensual relationship.
My question is why to we have so many teachers with such screwed up interpersonal skills that they can't have a relationship with someone their own age?