There is nearly always an image created of the '30 something ravishingly handsome teacher' and 15/16 year old smitten student with the ensuing tragic and destructive outcome for both. However, with the huge changes we now see in worldwide teaching and the surge in those returning to adult learning e.g. a teacher having a class of 25-40 year old adults for example, I see nothing wrong with it.
The stuff in regard to cheating and favours of a certain kind I see as totally separate moral and/or ethical issues entirely. 8)
I teach adults and I've had relationships with students. I don't mark their exams, so there haven't been any issues to worry me.