The following is from my 1709 copy of Collections and observations methodiz'd concerning the worship, discipline, and government of the church of Scotland.
"Adultery is the violation of another's bed... and is committed by a Married person's lying with an Unmarried, or the Unmarried person's lying with one who is married. If the Woman with whom the adultery is committed was at that Time living as a common Whore, and the Committer was a single Man, and knew nothing of her being Married, his punishment should be moderated on that account, but if the Man was Married, the Crime is the same, whether the Woman was a Whore or not, it being still a Violation on his part." That would seem to suggest both parties are guilty of adultery, the married and the unmarried.
The same book says "Fornication is committed by the carnal knowledge of unmarried Persons."