I am finding it more useful as a mental exercise to imagine situations that involved some hierarchy, where couple privilege was either absent, or just not a problem or issue.
My former poly group:
I considered myself solo poly, practicing egalitarian with three men and one woman. My polycule was a four person quad with a married couple, a boyfriend, and me, and I also had the Zen Sadist all to myself external to that. The women in the quad were more romantic/sexual to one another than the men were to one another, but we were all very close.
The married couple was assumed in an easy sort of way to be primaries for one another. They were the only nesting couple, they went on trips together, they had ties to one another's families. No kids. As someone who WANTED to be "solo poly" during our time together, and did not seek a nesting partner at that point... I had just broke up a long marriage and wanted time to get my feet under me, so I was resisting getting "too serious" with anyone...
So any "privilege" they might have had was irrelevant because I was not envying it, seeking it, grasping at it. I never once had any illusions about becoming a lifetime romantic partner to any of the quad, really. I was not looking too far ahead. Fire and Hefe went to Acapulco. I was like, "Have a great time! Can't wait to see the pictures!" I wasn't bummed or unhappy at all.
A lot of whether you perceive that you're being treated in ways that are unfair or like a second class citizen will depend on what you want in life. There are people who have swimming pools in their backyards...I would rather not, I think they seem like a lot of work to maintain. Some people have seagoing boats. I live in the mountains. I don't need a seagoing boat.
If the elements that make up "couple privilege" are completely extraneous to what the "third" is wanting and seeking in life, then they aren't likely to fuss about it. Not everyone wants to jump on the relationship escalator, or is at a point in life where that makes sense.
But at the same time, when we were all together, being kind of rebellious spirits, we did not hide our relationships. I never felt like a shameful secret. We were all affectionate to one another (still are, in many ways, very PDA types.) I would not like being anyone's secret.