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Diam0nd it sounds to me like you're overemphasizing the power of the sx instinct and failing to realize that sp and so can just as easily take greater precedence in an individual when it comes to relationships.
To say that Sx is the "relationship instinct" is kind of a misnomer in my view. Pretty much everybody on the planet is looking for a relationship, assuming they aren't already in one. What's more important is
why they want a relationship in the first place, and what they hope to get out of it. For Sx firsts this just means that their greatest drive for survival lies in having a strong and intimate energy-sync with another, while for say an Sp primary they'll want a relationship that better contributes to their own sense of personal well-being and homeostasis. I couldn't really say what drives So primaries into relationships since I barely know any So-firsts, but I would imagine something about seeking a relationship which better allows them to associate with whichever group they belong.
Furthermore, I find that it's more beneficial to view the valuing of instincts as lying on a continuum where each instinct is allocated a certain amount of focus, as opposed to lumping the stackings into simplistic discrete chunks of first -> second -> third. All of the research I've done on the subject suggests that all people value the three instincts to varying degrees, and just from an intuitive standpoint I don't see why each instinct would easily line up in clear-cut order like that. For example, I see both my sexual and self-pres instinct to be fairly close to each other with social falling flat near the bottom in terms of priority, so probably closer to 47% - 38% - 15% than a simple gold - silver - bronze. In this way, to say that all sx-seconds are most compatible with each other kinda becomes a moot point when its degree of valuation can be so varied, especially when compared to the primary instinct which will take higher precedence anyways.
For the record, I still have strong reason to believe that similarly-valued instincts get along best with each other, given what I've observed in my relationships with others. In my experience a pair with shared first and flipped second instinct can be engaging and comfortable for a while, but I find that the second instinct eventually grows to be more grating than beneficial for personal growth. The way that Sx/Sos try to engage me in a sort of "play with meeeee" mentality gets to be too much for me at times, makes them come off as too emotionally dependent.