That was when there were 2 people. Not 6 billion. We've filled it already. God also called us to be good stewards of his planet. Not having children certainly is an option.
Couples can adopt children.
That's certainly not being selfish. Actually, the planet could support far more people than there are now. Children starve because no one on the planet will give them food, not because there is no food on the planet to give them. Of course there are many local reasons for famine for example inefficient traditional farming methods, natural disasters and socialism but too many people on the earth isn't one of them. The U.N. has released three different population scenarios, each based on slightly different assumptions.