He was responding to a book by David Benatar,
Yeah. Not making it up.
Benatar's schtick is that existence is bad no matter how happy you might be in life, so it is always morally wrong to create more sentient beings. Benatar says that the solution to human misery is to have absolutely no humans at all.
Of course, the first thing that is popping into your head right now is the same thing that I thought: "You go first, Indy. We'll be right behind yez."
But if we are into making ridiculous suggestions to increase human happiness, I've got one that will be even easier to execute (haw!) than mass sterilisations or building global death camps.
How about we give both Singer and Benatar a deal. In exchange for a promise that they will never EVER publish anything ever again, they get a free lifetime supply of Prozac.
...and transport to a secure island in the middle of the Atlantic.
Heck, we can throw in a box set of all the Calvin and Hobbes comics too, just to show how nice we are.
H/T to Zach