You are not to commit adultery.

In the natural sense, this commandment forbids not only committing adultery, but also having obscene desires and realising them, and so indulging in lascivious thoughts and talk. It is clear from these words of the Lord that even lusting is committing adultery:

You have heard that it was said by the men of old, You are not to commit adultery. But I say to you, that if anyone looks at another man's wife so as to lust after her, he has already committed adultery with her in his heart. Matt. 5:27, 28.

The reason is that when lust is in the will, it becomes like a deed; for the enticement only enters the understanding, but the intention enters the will, and a lustful intention is a deed. More on this subject can be found in my book CONJUGIAL LOVE AND SCORTATORY* LOVE, published at Amsterdam in 1768. This contains sections on he opposition of [scortatory and] conjugial love (423-443); fornication (444a-460); types and degrees of adultery (478-499); the lust for deflowering (501-505); the lust for variety (506-510); the lust for rape (5-11, 512); the lust for seducing the innocent (513, 514); the imputation of either love, both scortatory and conjugial (523-531). All these ideas are meant by this commandment in the natural sense.

* A term to describe love which is the opposite of marriage love.