Are Infidelity and Adultery Synonymous?

So we were discussing this issue today about infidelity and adultery, all in anticipation of Lawyer X’s book, Infidelity, which should drop on Thursday. By the way that should be free for 24 hours on Kindle. And so I posed the question to Lawyer X: What exactly is the difference between infidelity and adultery? She was only happy to scroll down the screen and read me exactly what she had written in the book on that very question. Here is an excerpt from Lawyer X’s book explaining what is the difference between infidelity and adultery:

This might be a good time to drop the big question: What, exactly, is infidelity? What does it entail? Is it, for example, synonymous with “adultery”? From what I can gather, the two terms are almost synonymous but they are not quite the same thing. According to the Oxford Dictionary, “adultery” is “voluntary sexual intercourse between a married person and another person who is not their husband or wife.” So to be clear, you can only commit “adultery” if you are married. Unmarried people who are in a relationship – even if it is monogamous and even when they have a reasonable expectation that that relationship is exclusive and committed – cannot be accused of having committed “adultery” when they stray from their partner. YOU HAVE TO BE MARRIED IN ORDER TO COMMIT ADULTERY. Full-stop.
Thus, any unfaithful act by an unmarried person is merely an act of “infidelity.” Even if this act is sexual in nature. However, the reverse is not true. Married persons can commit both acts of infidelity as well acts of adultery. Indeed, one can consider that every act of adultery is also an act of infidelity but not every act of infidelity is an act of adultery.