Resident monsters will never leave, their colour is visible (where as a visitor is always all white), they will respond to your commands (God Mode, Left Click with no Tool equipped), and you can fuck them in Player Mode (in Player Mode, Left Click with no Tool equipped) The only monsters this might not work for is Direwolf and Arachne, since they were both removed from the game for the purposes of the Beta release. Also Lamia and Mermaid do not currently have sex animations.
p.s. sorry for the delayed response