If your boyfriend or girlfriend cheated we’d say “RUN”. Even if the relationship may not have been a full commitment, the idea that your partner felt the need to go and explore another intimate relationship or “hook up” may be a sign that he or she is not ready for the commitment. Or he or she feels that the commitment was not established and both of you are entitled to explore other relationships. This requires a conversation between the two of you for clarity.
Source: Mediation and Counseling Blog