I think it's a good point, that partnership means different things to different people. To me, a "partner" is someone with whom I am consciously cultivating a relationship with the intention of continuing to know each other more and more deeply, supporting each other's goals and development, and finding meaningful ways to include one another in our lives. Very similar to friendship, and yet more charged, more deliberate. It comes down to a combination of intimacy and commitment that's mutual and acknowledged.
Intimacy means a lot of things, and it often means sex. Yet, I've had friends with benefits with whom I have definitely not been in a relationship. Sex, therefore, cannot be enough to define something as a relationship rather than a friendship. Nor, I've found, can its absence be enough to "downgrade" a relationship to a "mere" friendship.
If a marriage becomes non-sexual, are the spouses still in a relationship? Would they be so even in the absence of the legal contact that binds them? I think that, in most cases, people would agree that they are. If we accept the idea of a non-sexual life partnership, then, to use the poly parlance, is it only primary relationships that can be non-sexual and still be relationships? No, I don't think so. A secondary partner is, in most cases, a good deal more than a friend, and it's not just the sex that makes it so.
Those are some initial thoughts, on a subject I'm still working out for myself.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.