It's become apparent from other discussions that one term won't fit all partners. In the hierarchy of primary/secondary/tertiary, I'm thinking we can try to find terms to fit each that aren't clinical and don't sound derogatory in any fashion.
As I posted elsewhere in this thread, I think "paramour" works for primaries. For those who differentiate between their levels of involvement, then, what would be a good term for a secondary? A tertiary?