Even good sex can become standardized. In this way, sex is like the rest of life. It's actually less than you hoped. For almost everyone who reaches middle age, sex and life become a customary enjoyment, an habitualized routine of pleasure, comfort, and pain that is consoling at best, and often meaningless. This is good.
Meaninglessness is a sign of growth.
When something becomes boring it means you are ready to go deeper. When you are humping away but still unsatisfied, you are ready for deeper sex. Just like any other moment of life. When you allow yourself to feel sex completely, you feel two things. On the one hand, your genitals are engorged, your chest is heaving, and your passions are inflamed.