Ghouls do not procreate with one another. They propagate their species by copulating with human females, some of whom are willing (having Ghoul relatives), and some of whom are not (most likely through a distaste for bestiality). Their offspring are born very much human, but as they become adults, they begin to transform into Ghouls. At that point, they usually tend to return to the Underworld to join their Ghoul brothers. Ghouls are carrion feeders; to them, a graveyard is a perpetual smorgasboard.