The best answer I ever heard to "Is it ok for teachers to have romantic relationships with their students?" comes with a bit of a story:

When I was attending Reed College, my ethics teacher would take one day out of each semester and field ethical puzzlers from the class. You could pose any question you wanted and he'd give you the ethical lowdown from various perspectives.

However, it was well known that he had married several of his students. As far as anyone knew, he'd never had any improper relationships with them while they were attending the college, but the marriages had happened soon after they'd graduated.

So on the day he was answering our ethical dilemmas, of course one wiseacre had to ask him whether it was ethical for a teacher to date his students. The entire class inhaled and the professor looked down at his shoes for a moment, then looked us straight in the eye and said "It depends on the circumstances. However, there is always a power differential between a teacher and a student which inevitably distorts the relationship."