A dream where you are having an affair suggests that you are suppressing your desires and needs in some aspect of your life, or you are seeking satisfaction or excitement in some part of your waking life.
To dream that someone is cheating on you implies that you feel insecure or are not receiving appropriate attention in some part of your waking life. Alternatively, it can suggest that you feel you are being taken for granted or not measuring up to the expectations of others in some area of your waking life.