For me, my primary is my husband. We are life partners, first and foremost. He has made it clear that he does not want anyone else to share that place, in my life or in his. No matter how important another man becomes to me, he will never live with me, or share finances or major life decisions. That is out of respect for my husband's preferences. That does rule out any potential partners who are looking for that deep of a relationship - but I am upfront about what I can and can't provide in a romantic partnership.
My secondary is my boyfriend. He is a very independent man who has never been married and never wants to be. He needs a lot of alone time and personal space, and is a major commit-a-phobe. It can be hard for a man like that to find a suitable partner in the regular dating scene, since most people have the standard expectation of a monogamous relationship that builds to living together and marriage. He prefers his "secondary" status, because he doesn't have the emotional skills or energy to devote himself fully to a relationship.
None of the people involved find the situation unloving or unfair. In fact, none of us would want my BF to hold the same position as hubby. BF doesn't want that responsibility (dealing with family drama, taking care of me when I'm sick, etc.) He just wants some companionship, some sex, someone to go out with and have a good time.