Falling in love with your sensei is a bad idea, not that people have control over these feelings. There are instructors that take advantage of new, awe-struck students. There are even cases in which the sensei might feel a sense of entitlement to their female (usually) students in the same way a lord of the manor may see female subjects as their chattel.
Some get off on the power relationship inherent in a student/teacher situation. I believe the student is the one who suffers most, especially since they are on another's "turf" and can be easily manipulated.
I've seen a sensei obviously hook up with one student one night, and the following day, ignore her on the mat and deliberately focus instruction on all the other females present, a few of which were his former conquests. The poor student would just look up with wistful eyes, probably wondering how badly used she's been.
In situations where the feelings are mutual and sincere, dating the sensei is ok. But usually, as someone had stated earlier, the student has no idea what she or he is getting into until it's too late.
Hats off to those who can continue training under those circumstances and overcome that barrel the ugly feelings of being used and lied to.