Are catholic priests supposed to masterbate?
No one is supposed to do that. Priests, being celibate, are supposed to refrain from all sexual activity as a sign of devotion. Note that this is only in the Latin Rite (the Rite most commonly practiced in the U.S.) since in the other 22 Rites priests can choose to marry. Priests do have confessors, and I suppose this is something they confess. Otherwise, self-control is a gift of the Spirit, so perhaps it isn’t as much of a problem. Those who have never tasted the fruit perhaps never long for the nectar.