Different people have different ideas about what polyamory is. Simply put, it is loving more than one parther. Non-monogamy is another term used. It doesn't mean someone has to be in a 3 way marriage to qualify. If a person wants to have a steady relationship and see others on the side, that is polyamory too. If they want to have 3 or 4 steady relationships, that too is polyamory. If people want to have sex with their friends, that is polyamory. Generally, people love their friends, that qualifies.
You assume Adult Friend Finder is all about casual sex, and you would certainly think so based on their advertising. But there is a whole blogging community there that encompasses about every possible lifestyle imaginable.
For a person into an alternative lifestyle, you don't strike me as being particularly open minded.