TL;DR summary: guy gets a vasectomy. Three months later, he meets a woman with whom he enjoys a casual relationship. She had other designs, however, and about four months into the relationship she plays the "I am pregnant" game, which our hero indulges with a bit of pleasure (knowing obviously the child is not his). After revealing her to be completely crazy and immoral, he performs an awesome coup-de-grace that leaves her sobbing.
Comeuppance! Catharsis! Woo.