The world isn't actually over populated, it's just very badly managed.
Our Agriculture infrastructure isn't even close to full production. We could easily feed several more billion people and have ample supply for animal Agriculture with current technology.
The world only seems overpopulated to some because 98% of the world's human population is trying to live on about 3% of the Earth's land mass (NYC, LA, London, Tokyo, Beijing, Sydney, etc).
If people want to have kids, good for them. It's not your place to say what their family should do one way or another.