Hmm. Maybe the zombies are repelled by the increased prolactin(theoretically the source of "post-nut clarity") produced in males after sex? Would explain why the girl was attacked once she got too far from him.
Hmmm, spent the whole first chapter developing just 2 characters, and one of them gets killed right off the bat for that cheap twist/shock value. Can't say I like it when they do this. It's a sign that any character can be killed off and replaced.