A couple of points/observations........For what it's worth, I think it's perhaps easier to deal with this issue when you are a little older (40's-50's) and more established in your primary love over many years and with a history behind you, moreso than just starting out in your 20's from scratch in a new relationship.......IMO.
My wife is lately having a very difficult time internally with our "V" and she can't quite put her finger on it...we've tried talking it out but have no concrete reasons/answers.....she believes her hangup to be around having to "compete" with the other woman in our "V" from a sexual or love standpoint. I have tried to reinforce to her numerous times that this is not a competition, merely different types of love, intimacy or sex that compliment what she and I have always had. I thought we had this issue addressed, but it recently reared it's head again. I sincerely would not have an issue if my wife wanted to have a poly relationship and/or sex with another man, if that's what would bring her the level of happiness/fulfillment she craves in her life. I would be happy for her. I suppose if I sensed that the other man was trying to steal her away from me, I might be upset, but I would have to trust that was not the issue. That trust I think can only be derived from time in a lengthy, loving relationship.