Unless you've been married for forty years, no intimate relationship is complete without a healthy sex life. Sure, they slow down and get into a rut over time, but that doesn't mean you're still not enjoying each other's bodies. Humans have needs, and sex is right up there for most people with breathing and eating. It's a joy that few ever truly give up.
But if the interest stops dead there's either a more serious relationship problem or she's getting her kicks somewhere else. A month or so without getting jiggy isn't a huge deal, but flat our refusal, night after night, is a bad sign. Sure, you could just be awful in the sack. But it's more than likely it's one of the other two options. Either one could be a symptom or a cause, and either one spells out T-R-O-U-B-L-E.