I mean, if I go punch someone in the nose, I'm controlling what I do with my own body, and yet I think we'd both agree that's not within my rights.

This is just a variant of the quarantine or smoking examples: you can do whatever you want with your body, so long as it doesn't affect anyone else.

What's your basis for this sharp bright line? The people are separable well before they typically separate.

One is completely dependent on the other before birth, and not afterwards. We-ll, actually, that's not strictly true: see astra_nomer's point about saving the life of the mother.

I also still don't see the connection to the legal issue. I see a moral issue here, certainly, but not a legal connection.