Friends with Kids

Julie and Jason have been best friends for years with no romantic interest in each other. He sleeps with someone new every few days, and she's looking for Mr. Right. Now in their thirties, they notice that their friends seem to lose all their good qualities when they have children - child rearing and the spark of Eros don't seem to co-exist. So, they decide to have a child together, share in child rearing, but pursue their own romantic lives. Things go well until he meets Mary Jane and she meets Kurt. Both seem like perfect mates. What could go wrong?