Nacho_mx - nobody said instructors aren't allowed to date, but dating the students, especially if it is a habit rather than a one-time occurrence (fell in love and got married), is not exactly admirable - because when the relationship fails, it is the student much more than the instructor that can get hurt.
In addition to the heartbreak that both individuals may experience, the student faces the choice to leave the dojo, or else continue to take orders from an ex (that's right, take orders - in most dojos you are dealing with a military-like heirarchy).
That's a vulnerable position to be in, not to mention miserable. Especially if the instructor takes up with another student.
Get the picture?