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That s the only thing I could think of, because the penis is innervated by the pudendal nerve (generally) and the scrotum by the genitofemoral nerve.
Neither of those are anywhere near the umbilicus in terms of nerve distribution (genitofemoral n. is physically close but has no rootlets that go into that area to my knowledge).
The only thing I can think of that would innervate the umbilicus would be the last T-L2 spinal nerves that innervate the lower flanks. Internally, I don t think the umbilicus is innervated at all past birth.