Originating from the horse breeding industry where "service" is a euphemism for bringing the stud to mount the mare. Same as tupping in sheep breeding.
2. To have sex with a mardy bitch to keep her happy, as in a mechanic servicing a car to keep it running smoothly.
Dick Greenleaf (Jude Law) on a boat trip: "Marge needs a service" - winks at his mates and proceeds below deck and give his sulky bird a good seeing to.