Sexual Encounters Between Married Men–Not Adultery?

If a man chooses to engage in activity with someone else that he could do with his wife for, but chooses not to, that’s adultery. But it’s not like he could enjoy a man’s body through her. Where’s the betrayal? That’s what my friend explained to me as he showed me one online ad after another from married men seeking other married men, just for a weekend encounter. After that, the happily married men resumed their conjugal relationships. Everyone was happy, getting what they wanted. No one was the wiser. Opinions?