Sex you don't want anybody to know about. It has to be kept on the DL because of any number of reasons: you're embarrassed (coyote ugly), you don't want your boss to know, you're a cheating bastard and you don't want to get caught, etc.
"Yo, Smiles, did you hear that Mr. Ed and Toto are having secret sex?"
"No way Ldog, that isn't happening!"
"Oh, don't worry, it is. Apparently, Mr. Ed is afraid his boss will find out. As if Quatals would give a shit!"