Being the other guy means being the man that is having an affair with someone who is in a committed relationship. Infidelity is painful for anyone, and when you engage in a relationship with someone who has made a prior commitment this can affect how you feel and how others view you. The commitment does not have to be marriage, today it is common for a couple to live together before marriage. Being the other guy shows character weakness and an inability to avoid involvement with someone who should be committed to another instead.