have an affair

have an affair (with someone)

To engage in a secret sexual relationship despite one or both participants simultaneously being married to or otherwise in committed relationships with other people. Did you hear that Rachel had an affair? Her husband is just devastated. Cliché though it is, yes, my husband had an affair with his secretary. I can't believe you're having an affair with a married man!
See also: affair, have
Farlex Dictionary of Idioms.

have an affair (with someone)

to have a love affair with someone. When I was 20, I had an affair with a rock star, and really made a fool of myself.
See also: affair, have
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • have an affair (with someone)
  • gang-bang
  • gangbanging
  • wear more than one hat
  • sixty-nine
  • you can't have your cake and eat it
  • have cake and eat it too
  • have your cake and eat it
  • have (one's) cake and eat it (too)
References in periodicals archive
92% Of Lebanese Say That "Married People Having An Affair Is Morally Unacceptable" Only 47% of the French say it is morally unacceptable for married people to have an affair but as far as Lebanon is concerned, the study states that 92% of Lebanese say that married people having an affair is morally unacceptable.
Half of France thinks marital affairs are fine, but what do the Lebanese think?
FLIC SAYS: Sadly, hardly anybody's "the type" until they have an affair. And it does sound as though you may have reason to be concerned.
Your Life: Relationships: LOVE CLINIC: I'm worried he's seeing another woman
Thank you, Ellen, for giving me a much-needed laugh that day at the pathetic irony of learning about "Ellen's Crazy New Life" as it was revealed how she juggles a successful TV career and strong lesbian relationship as well as apparently finding time to have an affair on the side.
The adventures of Ellen
Would you ever have an affair with your partner's best friend?
The Adultry Files Part One: GREAT BRITISH ADULTERY SURVEY