You can make arguments for it starting with "killing billions of people would definitely be bad" or "some fraction of those people will likely share my genes, which I have a biological drive to pass on".
It encodes a slight bias towards human existence being a positive thing for us humans, but I don't think it's the shakiest part of that reasoning.
Perhaps you're arguing as an illustration of the way this group of people think, in which case I understand your point.