I personally find myself in the same situation as garden peach in that I am not married to my primary , and we used to be monogamous and are razing a child together. I prefer no labels …..but she is comforted by knowing she is my primary or main relationship… I don’t really like secondary though sometimes have used it as a descriptor. My other relationship was with a married woman who I referred to as my friend for years with off again on again great sex. once she tried labeling it as something more and my friend got jealous and possessive ruining the whole thing including the friendship which makes me really sad. But I think my friends bad relationship with her husband may have had a lot to do with it though.
definitely wish there were better words than primary and secondary though one obviously stayed when there became conflict between the two