Sex is usually a pointer to the health of a relationship, so if other things are not in equilibrium, then sex will be off of the menu.
So in my case married 30 years + haven't boned the OH for several years, I got used tot he idea, and watched porn and wanked a lot.
Kids have a lot to do with it. Whenever I am doing a lot of childcare, I find my sex drive (which is normally through the roof) drops to almost zero. I've also come to the conclusion that there is a hell of a lot more than sex to a relationship, even if that's what gets you together. I think it's actually much easier to find someone who makes the sexual sparks fly than it is the other stuff.