It's pretty simple - there are too many people and they are increasingly living high consumption modern lives.
Breaking down a problem into it's relevant components and understanding how they interact is the right way to come to a simple explanation.
This is in contrast to "simple explanations" which are merely short, but not correct.
So it's pretty simple: Let's just kill all people and their children over-populating _your_ planet. Right? </cynicsm_off>
The point I am trying to make is: This mindset of yours is pretty dangerous.
So how do you want to enforce the reduction in reproduction?
That's also only half the issue. The other half is the consumption. This would likely be more achievable through taxes, zoning, and the court's (potentially flawed) interpretation of interstate commerce laws.