It's because people get old, if they've been married for a long time.
They're still having sex more often, than people who aren't married, who are the same age.
Discussion
Seems intuitive, but is it so, really? I was researching that years ago for a study that I haven’t written yet, and there are so many variables, but marriage seems to associated with greater decline in sex life. I suspect that it’s because statistically it forces to stay together couples who otherwise wouldn’t. But serious research is scarce, mainly because people tend to lie about this big time.