The original thread title: Experiences with FB or FWB only
(the use of "only" caught my eye - not "in addition to other relationships").
In the text, the OP specifically said that it was about "keeping outside involvements limited only to "sex (f**k) buddies" or "friends with benefits" status only, and not allowing these interactions to progress to relationships (bf/gf, romantic, etc)."
I took that "not allowing" to mean that it was about restricting any involvement to purely sex and that all romantic interactions, let alone relationships were verboten. So this isn't about people choosing a specific relationship to be suited to a FWB relationship, but that there were restrictions in place than the only thing allowed outside the primary couple was sex.
So based on my (possibly flawed) interpretation of it, I don't think that it is relevant to a poly forum at all, and tried to steer the poster to a place that they may get better answers about how to restrict a relationship to be sex only.
Based on my conversations with swingers, it is far broader than a club or an organised social event, although it can be that. It can be simple "swapping" (hard or soft), or organized threesomes. In other words a relationship that is only allowed to be for sex, even though friendships can develop.
And I put no relative value on poly vs swinging, either.
"Listen, or your tongue will make you deaf." - Native American Proverb