Hm, interesting point, though I wonder if this is correlation or causality.

On one hand, you could make the argument that if a couple has frequent sex, they will be happier and have stronger marriages (because of the sex).

On the other hand, you could argue that couples with strong marriages (i.e. good communication, shared values, understanding of one another, etc.) are naturally going to be having more sex. As a woman, I have to favor option #2.

I think in general, problems in the bedroom are reflective of deeper problems within a relationship. Sex isn't going to fix a lack of understanding, respect, etc. between couples, so they should work on establishing a healthy relationship first and the healthy sex life will follow. Just my two cents on the subject.