[michaelt]

  People who don't intend to have children subsidising
  people who do (who are on average already richer and
  more privileged) is deeply messed up.

Assuming you were born yourself, aren't you opposing something you yourself benefited from?

[zimablue]

It sounds good but it doesn't actually hold up...

So if being born means that I have to approve of any measure which causes more births, then logically I should be in favour of for example reducing contraceptive programs in Africa, etc.

Being born doesn't automatically force you to support population increase or be a hypocrite. Even if you had a moral obligation to maximise (total number of people that will ever live), then that number is probably larger (because of humanity surviving into the future) if we drastically cut population in the present.