If he exists, then he was conceived. If he was conceived, sperm fertilized an egg. If that sperm was part of a larger ejaculation, there were tens of millions of sperm involved.
For the saying to make any sense, you need to believe that a person is the sum of those tens of millions of sperm, and not just the one that successfully fertilized the egg. Otherwise, it wasn't a part of him that ran down the leg.
Not seriously at all, just pointing out the nonsensical nature of the saying. It's this or keep refreshing news about the election at work, and this doesn't fill me with dread, so easy pick.