How do I stay monogamous?

I'm sure you've heard the cliche "once you get married the stops." Why is that? I'm not currently married but plan on proposing to her in a few weeks. But the isn't there. We probably go at about 2 maybe 3 times a week. How can a woman sleep next to her man in bed and not satisfy him? I can understand is there is a disagreement and they both go to bed upset but that normally isn't the case. I guess my questions is: Are all normal/healthy women like that? If so, why? And if I'm experiencing this before marriage will it carry over for the rest of our lives? I've already discussed it with her and she says "I'm just tired" or "I had a long day at work." I don't want to be an insensitive but I'd also like to go to sleep without being ually frustrated.