General: penis milking
Images or animations in which some kind of milking machine is attached to the penis of a character (or characters) in order to forcibly extract semen, typically through vacuum pumping. Alternatively, this tag can be applied when a penis is being stimulated by hand in an udder-pulling manner and the semen is being collected in some container
Such images frequently include elements of bondage or additional machines that provide sexual stimulation, generally anal penetration, to other parts of the male's body. If the character is being milked unwillingly, then this is a form of forced orgasm, as well as rape, and NOT bondage.