I don't know if anyone can give a definitive answer, but various hypotheses and experiences were brought up in a discussion of monogamy and polyamory at Less Wrong here: http://lesswrong.com/lw/2ee/unknown_knowns_why_did_you_choos...

As long as you can learn how to suppress or ignore or never have jealousy, multiple relationships seem strictly better (= more fun) to me than monogamy.

In addition to suppressing and ignoring jealousy, another option is to learn how to acknowledge and deal with it. Just as fear is quite a natural thing and hinders people in accomplishing their goals, so can jealousy be a hindrance to happy relationships. Both can be successfully managed however.

  Courage is resistance to fear, mastery of fear - not absence of fear. - Mark Twain

I believe the same can be said of jealousy. Also note that unchecked jealousy is not strictly a problem of polyamory; it wreaks havoc even in monogamous relationships.

Yes, I should have also said "accept jealousy" as a possibly superior option.

Did anyone else accidentally parse (= more fun) as a strange prefix notation way of saying "more relationships = fun?"

If not, I apologize in advance for my insanity. ;)

Yes, and I have never even worked regularly with a language that parses like that..

In my case, this is why: http://lesswrong.com/lw/xl/eutopia_is_scary/3f8u