I think Launchpad just has a history of casually shacking up with anything. It's kinda implied in multiple episodes in the background where he has side adventures. And in one episode he lists as past romances "a friend, a confidant, a deadly ninja, a forbidden mermaid, a wereduck, a clone of yourself, … a Viking shield maiden, a talking cloud of energy that one time…" Launchpad just gets it all over with being a world traveling pilot.

Genuinely thought Della/Launchpad was going to happen until the finale. Guess he's still into alien chicks.