I dont much care for the terms but I understand their use in establishing a commonly easily understandable concise method of communicating meaning regarding something which can be complex to convey using other terms such as partner, boyfriend, girlfriend, lover etc.
I think my objection comes from the value which seems to go with the terms; that primary is more important and secondary sounds like some kind of afterthought. My relationship in a triad is not an equal one. In that they are married to each other, live together and although we share each other, enjoy shared time and intimacy together, I am very much separate to their unit together. However I don’t consider myself secondary in terms of value. Our relationship with each other as individuals is different and different again from our relationship together as a three.