This depends on one's philosophy concerning sex.
However, in psychology circles when we look at the health of the relationship, there are several halmarks we look for.
I look for:
1. communications: how does the couple communicate together.
2. sexual relations. How is the sex life
3. how do they argue
4. courtesies, how much respect and dignity to they show each other.
Communications I believe is a higher priority than sex. However, the sex charged relationship is the best to have. While people can be arguing and fighting and have sex, the quality is often lacking. BUT where communications and positive due regard exist, the sex is generally great. THAT is the best kind of relationship.
(note: sex is a form of communications)