Man of Honour
I never expected anybody to understand polyamory. But i will say that it's a lot more complex than you may think. Sometimes it's not a choice, sometimes you just are going to develop feelings for someone while you're in a relationship. Repressing them an not acting on them because you're told that they're wrong has a lot in common with being bi or gay. Sexuality is complicated. And you know, give me one good reason why not?
Sometimes you will develop feelings for others while you are in a relationship but normally the individual would decide whether they wanted to pursue those feelings with another or whether actually it was merely a short term thing that they could "get over" for want of a better phrase.
If both partners are polyamorous and wish to be so then I'm not going to say it's wrong but if it's something that one partner decides then it sounds an awfully like an excuse to mess around with the other persons feelings and indulge any whim as far as it can be achieved.
I can understand that the prospect of having more than one partner may be attractive but it's something where I think both partners have to be keen on it rather than it being propounded by one and grudgingly accepted by the other.