If a man commits adultery with the wife of his neighbor, both the adulterer and the adulteress shall surely be put to death.
Leviticus 20:10

By Biblical standards (not modern English nor American legal), adultery is a sexual relationship between a *married* woman and someone who is not her husband. A man cannot commit adultery with an unmarried, unbetrothed woman. It's impossible by definition. A sexual relationship between them might be sinful, but it is not adultery because she's not married or betrothed to another man.

Don't get mad at me. I'm just letting the Bible define its own terms.