When you're libidinous you're driven by your libido, meaning your sex drive. When you describe someone as libidinous, it's usually not a compliment, since the word suggests being out of control with lust and desire.
The proverbial "dirty old man" exemplifies libidinous gone wrong. It's not that lust is necessarily bad, but making a fool of yourself is. The word libidinous comes from the Latin word libido, meaning "desire," a word the world relearned from Sigmund Freud in the early 1900s. Freud believed that the libido — sexual energy — is one of the powerful motivating forces for people and that psychological health is determined by how well a person controls and focuses it.