Oh and I've noticed a lot of mixed feelings to the whole "primary" / "secondary" usage. Most of the consensus being that people don't like referring to anyone as "secondary", but that the labels are indeed useful in describing the technicalities of the relationship...
I would find it helpful to think of "primary" and "secondary" as describing the RELATIONSHIP, not the PEOPLE.
When two people have intimate feelings for eachother, a third entity is created that is separate from the two individuals. When 3 people are in a relationship there are two, perhaps three of these 3rd entities. Practically one may be given more importance over the other. This does not mean that one person is given more importance than another, just that that relationship, that 3rd entity, is given priority.
In all areas of life, knowing your priorities is important. Is your health a priority? Is your work a priority? Both are important and part of your overall happiness, but you want to arrange your life so your health is not going to get in the way of your work and vice versa.