As if David Letterman Wasn't a Big Enough Scumbag

he is actually scummier...

And the No. 1 reason David Letterman is a cad: The "Late Show" host brought the assistant he was sleeping with on Caribbean vacations with his wife and their young son, sources told The Post.

Letterman's wife, Regina Lasko, had no clue the comic was having an affair with his pretty, much younger assistant Stephanie Birkitt -- a $200,000-a-year employee -- when she was tagging along on the cozy, romantic trips, sources said.

Lasko, 48, "has to be really, really pissed" to learn recently that Letterman was so brazen as to have invited his paramour on vacations with their 6-year-old son, Harry, a source said.