And, I absolutely agree with all of that. My value as a person doesn't come from other's opinions of me or my relationships (which is one reason being unmarried entirely, being poly or mono, or having relationships with people I find attractive regardless of their sex all work for me).
But, I am super pragmatic, as well, and value what I invest into a relationship, whether it's as a primary, secondary, or whatever. While I realize that we can never be entirely sure what will happen in life, for me it's important to have some say over the dividends of what I invest. This is true emotionally as well as financially. So, just as i don't want to become deeply emotionally involved with anyone who has a partner that holds veto power, I also do not want to invest a great deal in a relationship where a metamour holds legal powers over my funds (and, if you have any kind of a savings account with a married person, in almost ALL states, their partner has a legal right to half of whatever is in the account and sometimes more, regardless of who's name is on it), access to my partner in times of crisis, etc., which--whether we like it or not--is part of being married (in the US). So, I would choose to not invest my time in what I would consider a primary LTR with a married partner because, for me, part of being a primary partner includes things like saving up for vacations, being part of decisions to move out of the area and the decisions and financial responsibilities that may go with that, taking care of one another if ill, etc. (been there, dealt with a metamour who was married to my partner, and got fucked so hard I still feel the reaming-- and that was just general daily life stuff, not an emergency or death).
But, again, that is my reasoning. For some people, those things are not an issue. Perhaps they make enough money they don't need to save up together to take a vacation or they have no interest in vacationing together, or moving isn't something they'd do regardless, etc. Everyone's needs are different. There's no argument that marriage, in the US, imparts rights that no other contract does, and that many of those rights cannot easily be overcome; but, in some situations, that probably is much less of an issue than it it in others.
I think another point in my perspective is that it no longer matters to me whether I'm a "true primary." I used to care about that. Now, I just take life one day at a time, knowing we can never know all the bad things that could happen to us, so why try to tabulate them?
Whether I'm right or wrong about being a "co-primary," the most important thing is, Do I understand my own value as a unique person? Am I a primary in my own estimation? If so, does it matter whether others (even my closest companions) recognize that primary status? Surely it doesn't matter whether society, church, and government recognize it. They can ruin my life, sure, but they can't ruin my esteem for myself. At least such is my philosophy.
The irony is that I do doubt my value as a person. But those doubts are mine, and I recognize them as such. I don't look to the companions in my poly unit to grant me my sense of value. That's all up to me.