I want to thank the OP for this post, it's given us all a lot to think about. DH and I see a lot of arguing over what is a want and a need and how feathers get ruffled quickly. For us, this made perfect sense. See we came from a mono relationship into poly. It's not the smoothest transition for mono couples. Especially when one remains mono. It's incredibly hard on DH simply because of all this 'need' talk.
See all the resources are 'only move into poly if you really want to!', meanwhile having been on and both been moderators on, poly/mono resource lists we see more and more people moving into poly because of only one partners 'need'. The mono partner feels trapped for so long because they get told they just need to work through their jealousy, they need to find compersion, they have no control anymore. The other relationships must just find their own pace and way! DH himself and I by proxy, have actually gotten abusive emails and comments from so called 'poly activisits' who will happily and rudely explain how 'unenlightened' DH is because of the rules and boundaries WE have set up for ourselves.
The point for us was this. A need can not be negotiated. You NEED x amount of water a week or will die. You can not negotiate this. You can't say, "Well, what if I just go with y amount for a while and see how it goes?" You will die. You NEED food, you NEED a place to survive the harsher elements. A NEED is nonnegotiable.
Meanwhile, poly is a want, a desire. You may greatly desire the ability to have more than one romantic relationship, even feel as though you are unwilling to sacrifice that desire. However, it is not a need. And when you tell a partner who has been under the (correct) assumption that the relationship you were in together is monogamous, telling them you NEED to be poly is akin to telling them, "I'm going to have other people, either with your consent or dumping you and getting it elsewhere."
It may come to that in a mono relationship, but it's a horrible, horrible, place to start negotiating and working together towards a solution. It is not a NEED, saying so is manipulative and holding the mono partner hostage. Then telling them to be happy about it because their partner will be happy! It is a great desire. So let's see how we can negotiate it into our lives. Those negotiations don't stop, they evolve as both evolve into comfort levels and better communication.
You will find DH and I RARELY give advice linked to poly resource sites for this reason. The people running those sites have had personal interaction with us that tells us they have no sense of empathy for people in situations other than their own. They overwhelmingly tell us we are wrong in how we negotiated poly, and that even with my consent and agreement, DH is simply not taking into account my NEED to be poly. All the while the NEED of DH to go slow and respectful to our relationship and his feelings is ignored.
That difference between NEED and DESIRE/WANT is important. One can not be negotiated.
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year