Here's another one. This one is mpreg but there's a good explanation as to why it's possible. It fits the bill for a post apocalyptic fic. I really love this story. It's a complete fic too.

The Killing Moon (http://poisontaster.livejournal.com/231867.html) by poisontaster - Sam is one of the few who can reproduce; should he be auctioned off to a stranger or stay with his family?