@Meera - To be clear, I never lie and I strongly imply I am seeing other people on first dates. However, I don't volunteer more information then that unless a girl asks. Mostly they ask for details by the second date, so it's rarely even an issue. The main reason I don't make a point to tell is that some girls just don't want to know. Not everyone I date is looking for something serious. I'm perfectly happy to accommodate a non-serious, occasional FWB's situation. Until I know what a girl is looking for, I don't typically volunteer specific details about my love life.
@Rainy - Thanks, that's exactly the kind of experience I'm interested in hearing about. In your case you were already interested in poly. I doubt that will be the case for most of the women I meet. I'm very curious what the best, and ethically sound, way to introduce new dates to poly. I'm fairly sure that most women consider discretion to be a virtue in a man, so I'm not really at the point where I want to add the poly-label to my profile. I also freely admit that my attitude on this may change once my girlfriend and I move in together.
Anyway, keep the stories coming. =)