If you are like at least some other women in long-term relationships, you might find that you have these dreams right before you ovulate. After all, Elizabeth Pillsworth, Martie Haselton, and David Buss studied women who were not on birth control and followed their monthly pattern of sexual desire for someone other than their mate. These researchers found that the women who were in longer relationships had an increase in desire for someone other than their mate as they approached ovulation.
So if you are having these dreams or fantasies about that hottie who got away, just wait. By next week (i.e., after you ovulate), you might just go back to dreaming about work and your kids again.