Dreaming about having an affair can symbolize your own repressed sexual desires that you long to satisfy. It can be a reminder to you to spice up your sex life and keep it interesting so that neither partner gets bored. Alternatively the betrayal could be a more personal manifestation of your subconscious leading you astray. You may be involved in a situation which you will find is not in your best interests.
If you dream about your partner having an affair this symbolizes your sense of insecurity and shows that you are worried about being abandoned or rejected. You may feel like you are being taken for granted, or that your feelings are not respected. If this is a long term relationship it may be a natural shift from the passion and romance of a new relationship to a slower, more mature relationship, or it could be that your partner is showing you less affection than normal. Alternatively, you could be feeling suspicious about your partner or about someonesomething else close to you and this is manifesting itself as a fear of betrayal in your dream. This kind of dream can often occur when you are facing big changes in your life and the stability you had is starting to fracture. The betrayal can also show that you feel sorry for yourself and may be comparing yourself against other high standards that you are afraid you can’t meet.
Also see Betrayal.