Exodus 20:14

Thou shalt not commit adultery, the word here used including not only the unfaithfulness of men and women within holy marriage, but every form of impurity directed against the holiness of God's institution of wedlock. Not only is every thought, word, and deed that is sanctioned and commanded within the confines of marriage forbidden outside of these bounds, but also every form of obscenity and every kind of familiarity which is connected with the sexual development of man.