I have an opinion on this, but as has been said, it's difficult, as well. What I think is this - if a teacher has a relationship with a former pupil, after they've left sixth form and are, therefore, no longer their teacher, then fair enough. It's perfectly legal and not incriminating, and so they have no fear in that respect. However, I do agree that, if a teacher is willing to have a relationship with an ex student, then you have to question what was going on in their minds while they were teaching the pupil in question. I've (and a lot of other people, for that matter) always seen teachers as more of another mother or father, in a way, people who pass on knowledge for the good of the future generation (it sounds corny and cliched as hell, but it's a truism, lol), and so I couldn't possibly fathom having a sexual relationship with an teacher. But, then again, that's just me, and as we all know, affairs between teacher and student have happened.
And I'm officially rambling, lol. Soz...