If you aren't planning on breeding him, I'd say to do it.
Of course it is unnatural. So is dog reproduction without any natural predators or food shortages (due to the undeniable human capacity to make garbage for stray dogs to eat).
The humane thing to do is spay/neuter, unless you want puppies.
Letting your dog run around "uncut" would likely end up with someone else (and their bitch) having to deal with the puppies later--never knowing which dog sired them--effectively making you and your dog "deadbeat dads".