Complication arises in that Scripture makes numerous mentions of men and women having sex (directly, or by inference) sans marriage. As with ruling classes using prostitutes, concubines, courtesans, and the like. And a strict reading and literal interpretation of Genesis reveals Adam and Eve were not in fact married.
Instead of blowing off "complications" why not try to resolve the command against adultery with the rules about sex with concubines or slave girls if the subject is sex outside of marriage.
How could it be both forbidden and allowed in the same book of the law unless sex outside of marriage is not the subject?