Thou shalt not commit adultery.

These commandments now [that follow] are easily understood from [the
explanation of] the preceding; for they are all to the effect that we
[be careful to] avoid doing any kind of injury to our neighbor. But
they are arranged in fine [elegant] order. In the first place, they
treat of his own person. Then they proceed to the person nearest him,
or the closest possession next after his body namely, his wife, who is
one flesh and blood with him, so that we cannot inflict a higher injury
upon him in any good that is his. Therefore it is explicitly forbidden
here to bring any disgrace upon him in respect to his wife. And it
really aims at adultery, because among the Jews it was ordained and
commanded that every one must be married. Therefore also the young were
early provided for [married], so that the virgin state was held in
small esteem, neither were public prostitution and lewdness tolerated
(as now). Therefore adultery was the most common form of unchastity
among them.

But because among us there is such a shameful mess and the very dregs
of all vice and lewdness, this commandment is directed also against all
manner of unchastity, whatever it may be called; and not only is the
external act forbidden, but also every kind of cause, incitement, and
means, so that the heart, the lips, and the whole body may be chaste
and afford no opportunity, help, or persuasion to unchastity. And not
only this, but that we also make resistance, afford protection and
rescue wherever there is danger and need; and again, that we give help
and counsel, so as to maintain our neighbor's honor. For whenever you
omit this when you could make resistance, or connive at it as if it did
not concern you, you are as truly guilty as the one perpetrating the
deed. Thus, to state it in the briefest manner, there is required this