This is just my opinion, and since I've only been in a poly relationship for a couple of years I don't think it carries a ton of weight, but here it is:
Some poly relationships are inherently primary/secondary. My gf cannot imagine ever living with/having children with her bf. She enjoys the time they have together, but she is thankful I'm the one she's living with and spending the bulk of her time with and could not have the same relationship with her bf. I'm sorry if I'm offending anyone, but sometimes that's just how a poly relationship works out. I'm not applying value judgments on different parts of the relationship, we could as easily call them relationship A and relationship B, but that can be construed as having a value judgment placed on it too. I'm not saying that all poly relationships need be labeled or designated primary/secondary etc, but calling then primary/secondary does work in some cases because it is an accurate description and everyone involved (at least in my situation) is comfortable calling it that. I'm not saying that it's not difficult and a lot of juggling on the poly partner's part (as I'm finding out). I'm the first one to give my wife full credit for having to balance two relationships and I'm understanding when I feel like my relationship needs/wants are not being understood or responded to, but there comes a point where it's not just a momentary pang of jealousy and it really starts to bug me and I have to voice my opinions or risk feeling resentment for her and her bf, which is the last thing I want.
The concerns and feelings of mono spouses of poly partners may seem like whining sometimes, but if there are hard feelings and they are articulated in such a way that they are not just venting, I would be very hesitant to call it whining and would be much more comfortable asking the question "so what do you want, then?" Again, I'm not trying to offend anybody but coming from the perspective of having to deal with my wife's business, all the housework, my own job, and all the various other domestic responsibilities when she goes to see her boyfriend (in addition to feeling compersion, which I do) can be pretty frustrating, especially since the same kind of weekend getaway type scenarios are nigh on unobtainable for my wife and I. Of course I want to enjoy that aspect of our relationship, and when someone else is getting it and not you, it creates frustration and can lead to jealousy.
Lucky enough for me I have my own relationship with a new gf and I'm now getting those weekend getaway scenarios. Unfortunate that my wife has voiced her frustrations that when I go away she feels like she gets stuck with all the domestic chores. I'm not sure I would call it venting or whining on her part because I realize that yes in fact I am skiving off all the weekend chores to go snuggle with my gf. I'm glad she told me, because now we have both been on both sides of this situation, and it has created more understanding for both of us. Now when one of us goes away for the weekend we get as much of the housework done on Thursday and Friday as we can and that way whoever is going away doesn't have to feel guilty about leaving behind the dirty dishes, and the person staying home for the weekend can enjoy having the place to themselves instead of making it livable again.
I'm not in any way trying to say that my wife and her bf have it easier than me, heck, they seem to have more relationship frustrations then she and I do! As for the dinner/dessert analogy, I apologize if I offended anyone. It wasn't my intention to place value judgments on different positions in poly relationships. I'm just trying to describe how the relationships are different, not that one is more valuable than the other. I'm not trying to say that one is better than the other, they both have their own set of frustrations and disappointments and joys and satisfactions. Sometimes you just want what the other person has, that whole "grass is always greener on the other side" fallacy. Again, my apologies if I offended anyone; I'm just telling it how I see it.