Well, if you have just the one 'real' child you will have replaced two people (you) with just one, in due course. That's an effort by you to achieve a net reduction in the world's population. If you have two, you are just replacing two with two, keeping it level. No problem, then.
Must say, if your wife wants a child and you don't, it would be no surprise if she cleared off. You are denying her a fundamental right. Why , in doing so,you justify it for the sake of starving children elsewhere, now or in the future (that being the essence of your argument ) is not immediately apparent.