A question about sex and relationships.

In a relationship there are two basic componants: sex and "the relationship" part. What do you do if one is missing? Better question: what do you do when one is missing, but the other one is front and center! Now being in a relationship where there is not a strong connection (relationship part), but really good sex, is almost always considered "just sex". But what happens when there is a really good relationship, but no sex? Is it considered "just friends"?