That is to say a woman who is married is the person with whom adultery is possible. Both for her and the adulterer. But it is not possible to have adultery with an unmarried woman. See Chronicles I in the second perek, verse 46

 Adultery means not to have sex with a married woman , but does not mean a man can not have many wives [or even girl friends.] For example in Chronicles I in the second perek, verse 46 we find Caleb ben Yefuna had a few wives and a few girl friends. And most rishonim go with this. [See the Ramban/Nahmanides and the Raavad.] The Rambam however forbids a girl friend but only as an isur asey איסור עשה [a negative command derived from a positive.] But all other Rishonim go with the Ramban that it is allowed.

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