Cross-posted from RH Reality Check. "Are we going to talk about sex again?!" screamed my 12-year-old son, Nick, as he ran down the stairs, away from me. That was five years ago, and I had just sat down with him to have one of our father-son talks, this time about sex and sustainability. Now Nick, a senior, is preparing for college at the same time that the global community is preparing for its own important landmark: The United Nations predicts that on Oct. 31, world population will reach 7 billion. The confluence of these two events gives me reason to …