Although the vast majority of couples stay together after they have a baby, some rare couples find the stress of caring for a newborn is too much for them.
You should also realize that all couples disagree at times and argue with one another. At times they will even yell things they don't mean. It is between the two of them, and can't be interpreted by an outsider who happened to overhear (evesdrop on?) the interchange.
Still, why are you stalking this couple? They have been together for years and will likely stay that way. I think it is a very good idea for you to move well away from them to help you get over this obsession with a man who is already committed to someone else. You need to move on with your life and forget about him. Your energies should be directed at your own friends and hobbies, not at thinking about someone you can't have.