I've seen a lot of problems occur with dating in the Dojang. Even when I was single I kept it all business; I learned from my instructors mistakes.
I have seen/heard of situation where instructors use there precieved power as their primary mechanism to pick up chicks. I think that this is disrespectful to the arts that they represent.
I don't think that students should go to a MA class with the intention of "checking out the scenery" either.
However, I have seen a few situations were people meet at the Dojang and really hit it off. I have seen one circumstance where instructor/student met, and are now life long partners.
So, to each is own, I guess. If they aren't acting inappropriately in class, or using class as a pick-up joint, then I guess it's O.K..
Every situation is different, so it is too hard to come up with set "rules" in my opinion.