That's not the point, though.
John's kids are hypothetical. But so is John. As is Harry.
As is a 500 year old Aubrey de Grey.
Rights are what people decide they and others should or shouldn't be entitled to based on a consensus of what most of us consider to be, you know, morally right.
If there was consensus on the rights of people yet to live (who may or may not ultimately be born) there wouldn't be differing views on pro-life/pro-choice - and the terms would likely not exist.
To come back to the crux of your argument, though - I don't believe our John is any more responsible for a single potential offspring that doesn't come to pass than a father is for every sperm that doesn't fertilize an egg. That would be, as you've said, absurd.
My point was that suspending the natural cycle of life in a way that prevents pro-creation is, in my view, morally wrong. Something I imagine we disagree on.
To discuss Johns hypothetical rights when he came into existence at point B makes sense. At point C he never did so its meaningless to discuss his rights. At A its possible to discuss B in context of B but impossible to be indebted NOW to entities in contexts in which they don't exist.
The breakdown is where we reason from B to A and assign every hypothetical being that could have possibly arisen the privileges of an actual existing person in A. B's possibilities are only reasonable in context of B.
To imagine otherwise we are stuck with accepting the murder of a googol potential slightly different children that could have resulted rather than the relatively few we had. Only slightly less absurd we could be morally obliged to have as many children as possible even if the result is that they are on net less successful and more miserable to avoid the greater loss of existence.
Reason it out yourself and consider the plethora of absurdities.