Just speaking for myself, I don't believe that your position is obsolete or intolerant. You aren't up for poly. You know that and are clearly stating that. I commend you for that. There is nothing at all wrong with wanting a monogamous relationship style. Polyamory isn't more evolved, it's just a different style.
That said, you do come across as rigid in your thinking which could be construed as narrow minded. In my experience, the thing about life is that it's ever changing... as are we. We change, grow, evolve. Things that were once unpalatable to us may become palatable or even preferable later. For that reason, I prefer to take a more open stance. "This is who I am now. This is what I want now. This is why I want what I want." As opposed to absolute statements like "never", "absolute certainty", etc. Of course, that's just my opinion. Take it or leave it.
As to my comments on possessiveness, they were based on this statement you made: "Trust me, I've been called "possessive" more times, by more people than I can count. I really don't see myself in that way. Rather, I state clearly what I'm after in a relationship, and others can choose whether or not to accept it." Based on that statement, I suggested that you take their comments into consideration and really look at your behaviors. My comments were not about your handling of this situation with your wife but about this statement (which also sounds very rigid and inflexible.)
Again, just my opinion, but since I'm 50% of a dyad relationship, if I want to improve the relationship, it's in my best interest to really look at my own behaviors, communication style, and treatment of the other person. If I change that which I can control (myself and only myself), then it will affect the relationship. Expecting the other person to do all the changing and compromising is a recipe for disaster.
I do understand where you're coming from, however, I think it's a given that things could change over time. We've always had the ability to communicate about them when the times do come. However, sometimes, when one person states that their feelings might change on a topic (such as this one), there might be the assumption that the other will be just as accepting. I honestly don't think that my wife was prepared for my lack of change on this, and was probably upset by that.
I've always been willing to examine my own behaviors. If I'm not acting appropriately, then I do my best to change that. If I'm unaware that I'm acting that way, then I ask that people point it out to me. I'm not immune to criticism, nor to I avoid it. I know that my thinking comes off as rigid, but I cannot alter my feelings on something that I simply cannot bring myself to accept within our marriage. If I don't ever see myself as being open to polyamory, then I'm not going to say otherwise. To do so would be a disservice to both of us as it would give her a false sense of hope, and bring on undue agony to me.
We have a mutual friend that we're both really close with. I've known her for as long as I've known my wife. While I do find her physically attractive, and I do care enough about her to be willing to do anything to help her in a time of need... Sure, in the future, deeper feelings/desires might develop, even if it's 20 years from now, or they might not, no one knows... But if you were to ask me right now if I were to ever consider sleeping with her, or feel anything deeper than a friendship or brother/sister type relationship, and my answer would be nothing more than "no".
And you're right, one can only work on themselves in a marriage. And honestly, it's felt as though I have been the only one trying for months... Like I said, the ball is in her court to make the attempts on her side as well. I'm very willing to start fresh & leave this whole, unpleasant year in the past where it belongs, but I can't be the only one doing so, if this is to work. Yes, I know that sounded rigid too. But, like I've also said, I cannot be part of a marriage involving polyamory. It's a deal breaker for me, if it's a must for her, then she knows what to do.