Is flirtation wrong while in a relationship?

I've been with my boyfriend for three years and we love each other a lot. Last year we broke up for a few weeks and in that time I got it together with a guy I used to work with (there was always a bit of a spark between us) but obviously that ended when me and my boyfriend got back together. Lately I cant help thinking about that other guy and what could have been. We still talk on facebook a lot and I still fancy him. Is this wrong??