There's a difference between an infant, which is already a living, breathing human being & the sperm that are expunged in masturbation. Even if those sperm could have been used to produce more humans, there's no way to prove whether or not they would actually. The woman could fail to conceive, for example. If you wanted to make a law against masturbation, you'd also run into the problem that there is no victim, just the probability of someone that might have existed at some point maybe. I also see a conflict here with autonomy. Can we require people to turn all of their sperm into humans? They didn't choose to produce sperm; it is an accident of biology. On the other hand, people do choose to have children (usually); it requires conscious choice & effort (excluding certain exceptions like the female rape victim).