What’s the punchline?

“Three childless couples go to see a movie about a future dystopic world where women can’t give birth, but then a woman is discovered who can . . . ”

Actually, Children of Men was great in that 1970s everything’s-gone-to-hell way. Even the opening title reminds me of that genre. I was totally entertained. Still, the natalist theme was up for discussion afterwards. How could it not be?