Most Helpful Guy
Assuming he's close to your age, mid-to-late thirties is usually when the male sex drive starts to diminish anyway. Perhaps he's not getting enough exercise? Try taking him to the gym with you or something. If you start putting a lot of effort into getting into shape, he will probably follow your lead, and the sex will improve.
That or he wants some sort of kink that you are unlikely to provide, so he avoids sex rather than be disappointed.
Most Helpful Girl
In my experience the 2 year mark is generally the time sex becomes less frequent. The relationship isn't as exiting & it get's to feel like a bit of a chore unless you're really in the mood &/or you's just feel more comfortable to just sit & enjoy each other's company.