So when any evil is forbidden, not only the outward gross acts, but all inward acts and degrees thereof are forbidden likewise: as under killing, provoking terms, wrath, anger (Matt. 5:21-22), under adultery, wanton looks, lustful thoughts, etc. (Matt. 5:27-30). Now, all things comprehended in a command (though not expressed) are jure divino.

JUS DIVINUM REGIMINIS ECCLESIASTICI, 81.