If the dreamer is the one being unfaithful, then he is dealing with feelings of guilt over an apparent betrayal.
If an actual lover is the one being unfaithful, then this indicates the dreamer’s insecurities over the relationship.
The partner may or may not actually be unfaithful, but either way the dreamer needs to deal with her feelings.
If an unknown figure is the one being unfaithful, then the dreamer needs to be more discriminating with regard to who she calls friend.
Astrological parallel: Mars/Pluto combinations Tarot parallel: The Devil