This is an interesting post, but you're a failing to distinguish between becoming a member of the opposite sex, and becoming a typical member of that sex, or the person that you'd have been if you were born with the same brain but different chromosomes.
The former has a much broader range of outcomes, and there's no unique set of characteristics or experiences that define 'man' or 'woman' - just a bunch of correllations.
I wonder if the Superhappys could be persuaded that instead of modifying us to not feel pain at all, we could be modified to have the ability to feel pain switched off by default, but with the potential to be activated if we so chose - that would avoid their concerns about non-consensually inflicting pain on children who hadn't come to the philosophical realisation that it was worth it, but would still allow us to remain fully human if that was what we actually desired as individuals given the choice.