I think the issue here is of instructors abusing their position or making a habit of this kind of thing. I was told by my sensei that martial energy and sexual energy are very different and the latter has no part in martial arts training. This was a real issue for his teacher as well. However, my opinion is that if feelings are becoming serious, i.e. both people really like each other, they should go for it. You might only get one chance at happiness.