Idiom Definition:
  
    
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"to have an affair (with someone)"  
    when a person, who is married or in a committed monogamous relationship, has  sex or an intimate relationship with a person other than their partner.
     
Synonyms:
adultery, cheat, cheating, entanglement, extra-marital, illicit, fling, goings-on, hanky-panky, infidelity, intrigue, liaison, love, matinée, relationship, romance, thing, two-timing, unchaste, unfaithful
 
 
Both of these people are married to other people, therefore they are 
having an affair. - - - - ->  
