1) Adultery isn't a crime in the US
2) Cheating on someone (assuming you aren't married) isn't the definition of adultery
3) ..its having sex out of marriage. So if you aren't married to the parter you are cheating on, you were also committing adultery with them as well.
Obviously, I realize it's not a crime by law, I was using the 3rd definition of the word crime (3. A serious offense, especially one in violation of morality).
I was using adultery as a blanket term to refer to cheating on both an unmarried partner and a married partner. They are both despicable acts, but especially actual adultery.