The student is an adult and so is the instructor - if they then wanna go home together afterwards and make babies then that's their business.
The reality is a little more complicated than that. Both are adults, but there's a relationship of power that kinda screws up things. And by "screws up things", I mean "to the degree where not even those involved could draw the line between legitimate feelings and abuse of power."
As long as you have institutional authority over someone, you can't have institutionally sanctioned affairs with that person. You can, however, drink beer together.
and, for those completists who insist that the only law is written on
tombstones.