At work, my batchmates (there are only six of us) started pestering me and asking me all sorts of questions about what kind of girl i would like.
I normally have a set of answers that would pretty much negate a lot of their choices. For example, I insist on a slightly nerdy but highly cultured lady who would still be willing to go to the… crowded part of town to eat at some really nice restaurants. it makes it harder for them to find a person who would like my particular brand of humor and quirkiness.
However, one time as we were waiting for our trainer in the office, they started bandying around the idea of a female professor who they thought was perfect for me. eventually they said something like "If you don't want to go on a date with her, you have to give us a real reason why."
So at that time, i decided to tell them, so i chose "Okay, but i promise it will be unlike any reason you were expecting. The reason I won't go out with her is because I already have a boyfriend."
They were floored. One of them shook hands, and all the girls hugged me. The other guy didn't react much because that meant that all the teasing would swing right back to him. Two days later they insisted that they meet my bf. They did, and they were happier because then I could share more of my life outside work with them (i spend a lot of time with my boyfriend).
It all turned out well, I think.