There is always the possibility that such a dream may be a warning of actual waking-life infidelity--but even when this is the case, it's also almost certainly a symbolic picture of some way in which the dreamer is "betraying" him or herself. At one important level, the people in our dreams are almost always symbolic of aspects of our own character and personality. When the dream characters look like people we actually know and relate to in waking life, it is likely that the "dream people" are metaphors for the parts of ourselves that we project onto those close to us in our waking life.
For this reason, if I dream that my husband is cheating on me with a waitress at the roadhouse on the highway just outside of town, he may actually be having such an affair. But even if he is cheating, the dream is still a clear and compelling picture of the (masculine, less-than-totally-honest) part of myself that I project onto my husband in waking life, and how I betray myself by allowing that part of me to take advantage of the (feminine, wanting-to-please) "waitress" part of me, in a "conspiracy" to prevent myself from being as strong, conscious, creative, and in charge of my own life as I am capable of being.
© Jeremy Taylor, 1996