Limiting the size of people's families is not taking away their sexual rights...it's dealing with their reproductive rights.

And it goes for everyone, of every religion or sexual persuasion. 

We have climate change, and water is going to be in very short supply in some parts of the world. Human beings are sometimes not very altruistic....have you not heard of the Selfish Gene? Bees in a hive work in harmony, for the good of the hive. We humans should work in harmony for the sake of our planet and for the sake of Humanity, but where resources are scarce the selfish gene comes into play.

As to how it would be done.....there could be rewards for those who limit their families, and penalties for those who don't. Simples.