A study published today in the British Medical Journal surveyed older people. They found that those in better health enjoyed more sex and were more satisfied with it than those in poor health. This was true for men and women. Indeed, interest in sex among middle aged and older men in the United States has increased since 2000.
Avoiding hypertension, diabetes, and high blood lipids (cholesterol and others) helps to avoid blood vessel damage. Healthy blood vessels promote blood flow to vital areas. Good exercise capacity and muscle strength improve all sorts of function.
I suspect that good health promotes good sex, but the reverse may also be true – what better stress reliever?