in my experience ive always found that bringing in 3rd partys only ever really works if the r/ship you're in is more of the casual kind.
in any close r/ship ive been in partners have always been happy to go ahead with three-four somes, seeing others seperatly etc but and there always was a but they could never handle it afterwards despite how much we'd talked,who'd organised it or how many agreements were made.
its all very exciting and fun at the time but it can be very difficult for most people to sit in the cold light of day afterwards and not have those tiny little niggling insecurities,jealousys and doubts creep in ...does he/she find them more attractive, does he/she find them more exciting, is he/she ever going to only want others involved now, what was his/her real reasons for wanting to do it,was it really just for fun or is it because he/she is bored with me, he/she was very comfortable with it all has he/she ever done this without my knowledge, etc etc and most cant handle that it just builds up and builds up until resentment anger accusations and guilt kicks in then all hell can break loose!

thats what ive found anyway, some people do it and are very happy that way but in reality most just dont get the outcome they were hoping for.