Shaya asked me to comment, as we are one of the few FMF V's here. We're 3 years in, with me starting as secondary, but the relationship becoming rather serious. Our relationship does have big problems.
I don't know many poly people and I don't know which configurations are stable. I can only say what fits with my experience.
Actually, I like what Spork wrote on post #4 (I'll take out a few citations that fit my view
):
The more orgasms I have, in general, the more I WANT to have. I might be fucked into exhaustion sometimes, but it really does not take long before I'm ready to go again
I believe the effect is real, but doesn't really apply to us.
Also, it could be balanced easily if the man had a higher sex drive than the women.
Even though men and women often both work, women are still often the head of the domestic sphere. ...
Now bring in another woman, and either she is forced to be submissive to the wife and live a sort of twilight half-life of domesticity like a servant...or there can be conflict.
This is certainly a factor
for us. I'm in fact not very domestic, but I refuse to accept the pre-set order of 'how things are done' in the older relationship. In addition, our age difference is 15 years - and I'm carrying over all the dysfunctionalities I have with my mother into the dynamics. Cohabitation is not an option for me, not only because of jealousy but the clash about 'who's in charge' too.
(Now, there are no doubt a lot of women can cohabitate just fine with a woman and a lot of man can't cohabite with another man, and I have no idea about the percentages. Actually I don't think there will be much of a difference, just maybe the typical type of conflict won't be the same.)
Now if you take cohabitation out of the picture, you avoid a lot of that, but I think that so many people are wired for escalation ...
the man who finds The One, the woman he doesn't just want to chance might take his seed, but the one he wants to provide for and protect...he is perhaps apt to either continue to make her The Priority and treat a secondary girlfriend as too casual and not give her the degree of love and consideration and energy and connection that she needs to remain happy long-term...or else fall into a more "serial monogamy" route without meaning to
While Idealist is very egalitarian, he still has tendencies to both extremes Spork names. (Though I'd be surprised if women hinges didn't have them.)
Continuing on that thought of man being the "protectors"
A V is essentially one poly person with two monogamous, but poly friendly partners. Traditionally, women are more the nesting sort. Therefore, in a FMF there is at least one F partner who is out in the cold, so to speak. I don't think men need that sort of security as much.
It's one of the few more-or-less-biologically given differences, that women are more vulnerable when pregnant and fall out of the workspace when they have kids, so indeed in that period of our lives we need more protection. This feeling I think carries over also to times when it's not needed that much. Furthermore, in me, the (cultural?) idea of the man as a breadwinner is very deeply rooted. So yes, I do think women need more home-security. And, I AM posing the question "how do I have family in a poly relationship? can you provide, with money and care, for me and the kids?"
If something breaks us up in the near future, this will be it.
So my guess is, if FMF triads are indeed more instable, blame the biological clock and the idea/state of affairs that the man provides for the household.