Use lascivious to describe a person's behavior that is driven by thoughts of sex. If someone gives you a lascivious smile, they've got only one thing in mind.
Latin-based lascivious and the Old English word lust both share the same Indo-European root las- "to be eager, wanton." The much older word lust originally meant "desire, pleasure" and over time developed to mean sexual desire. Lascivious, on the other hand, entered the English language in the early 15th century complete with the meaning "lewd, driven by sexual desire."