I dislike "primary" and "secondary" because they are comparison words... and I try to avoid comparing lovers, who are unique and wonderful people. Suppose you were single and then hooked up with someone... you'd never call that person your "secondary" partner if you had no "primary." The words themselves imply a pecking order.
No, the girl I hooked up with might be my "girlfriend" or "FWB"... or... whatever. I don't see how that changes if I have other lovers as well.
"This is my wife Alice, my girlfriend Betty, and our special friend Charlie." Noncomparitive and more descriptive.