I have this rule, although it's kind of moot given my marital status, against dating within my field. While Trillian and I met in a class that was required in my field, it was only peripheral to hers. Some of this is due to my competitive nature; I would find it hard not to compare--grades, abilities, etc. Since my wife was using an aspect of my field as a tool in her field, not as the central focus, we could study the same material without encroaching on each other's territory.
There are several couples in my department, and I always wonder at their ability to balance their work and their relationship. Perhaps some of them are less competitive than I--though that's definitely not the case for all of them--or they are able to create a safe space around their relationship. I respect that most of them are committed enough to work through the rough spots.
And then I was introduced to a whole other aspect of the intra-departmental relationship. Recently, my best friend in the department--who is straight--was out with other students for a social event. Another student, who has made it quite clear that she embraces many forms of sexual expression, hit on her... quite insistently.
Now, after I had a hearty laugh, I realized that this is an aspect of my rule I had never considered--the one night stand. I'm pretty sure I had not consciously formed a corollary to my rule on this issue, probably because it is so obviously a bad idea. Especially when you will have to face this person around the department on a regular basis. Especially when the prey makes it clear that s/he's not interested and you keep pressing--wouldn't you get the idea that anything that might happen afterwards would be a result of either the pressure or too much alcohol and a major source of regret?
But maybe I'm just old-fashioned that way.