Thinking back to the triad I was in, my partners tended to get "cowboyed" a lot more than me, but part of that was probably that living with TWO beautiful women made it pretty clear that I had little motivation to move along.
Meanwhile, each of them had me & her, & I wouldn't be surprised if some of their dates either discounted their girlfriend as "not serious" (or at least "not competition") or harbored hopes to squeeze me out & fulfill their FMF fantasies.
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In this discussion, I can't overlook the recurring "terminology creep" problem: what is meant by a relationship that is "long-term" or "serious"? I don't get much past "anything but an intentional one-night stand with a stranger."
Even then, anyone in Monogamist society can claim that an endless series of one-nighters is "looking for The One" & therefore reasonable as "serious" (&, for that matter, "Romantic").
Or, heck, what
exactly is meant by "a relationship"? As I've said, we each have "relationships" with dozens if not hundreds of people: family, friends, co-workers, the bus drivers I usually ride with, the staff at my favorite shops & restaurants, the head librarian....
To get out of that one, we paste on one word or another -- intimate, sexual, Romantic, committed -- all the while pretending we all agree on what THOSE words mean when in fact few of us have ever attempted to define them for ourselves. If I were to apply my personal standards, I can argue that there's a LOT of ongoing sexual relationships in the world that are questionably "intimate."
Unless we can all be certain that all of us are using the words in the same way, we're probably talking past each other.