[munksbeer]

I don't see how bringing new souls (whatever those are) into existence should naturally qualify as a good thing?

Perhaps you're arguing as an illustration of the way this group of people think, in which case I understand your point.

[dmurray]

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.