Say that the group of like-minded and mutually friendly extreme masochists existed first, and wanted to create their mutually preferred, mutually satisfying sadist. Do you still have a problem with that?

That seems less worrying, but I think the asymmetry is inherited from the behaviours themselves - masochism seems inherently creepy in a way that sadism isn't (fun fact: I'm typing this with fingers with bite marks on them. The recursion is interesting, and somewhat scary - usually if your own behaviour upsets or disgusts you then you want to eliminate. But it seems easy to imagine (in the FiOverse or similar) a masochist who would make themselves suffer more not because they enjoyed suffering but because they didn't enjoy suffering, in some sense. Like someone who makes themselves an addict because they enjoy being addicted (which would also seem very creepy to me))

To be consistent with this, do you think that parenting of babies as it currently exist is problematic and creepy, and should be banned once we have the capability to create grown-ups from scratch?

Yes. Though I wouldn't go around saying that for obvious political reasons. (Observation: people who enjoy roleplaying parent/child seem to be seen as perverts even by many BDSM types).

If David had wanted a symmetrically fulfilled partner slightly more intelligent than him, someone he could always learn from, I get the feeling you wouldn't find it as creepy. (Correct me if that's not so). But the situation is symmetrical. Why is it important who came first?

I think creating someone less intelligent than you is more creepy than creating someone more intelligent than you for the same reason that creating your willing slave is creepier than creating your willing master - unintelligence is maladaptive, perhaps even self-destructive.