I am not married, but I've been dating my gf for 6 years. We plan to get married in the next couple years. I've found that there are peaks and valleys. That happens, and communication is important to work out of ruts. It's best not to let the valley slowly decay into a physical and emotional canyon. But for alot of people, by the time a canyon has formed they are already neck deep. They have kids, shared assets, shared social circles, ect. It's difficult to untangle all of that because your sex life is bad. For some people it explodes into cheating or they divorce/break up. For others, they find other fulfilling aspects of thier relationship. Its not a simple issue for alot of couples, especially those who have been together for a long time.