GalaGirl: That is some cutting insight!
Evidently I'm straddling two worldviews. In the old one, where I joined my wife, the "til death do you part" is still taken somewhat seriously. (Serves me right marrying a half-Sicilian Catholic!

)
Of course people grow and change, and rational people should be able to accept and deal with that gracefully -- if not constructively.
In the new/evolving worldview a greater diversity and fluidity of arrangements is accepted. Properly and explicitly delineated, that allows for greater happiness and fulfillment for everyone (or, in game theoretic terms, a higher potential social efficiency frontier). I suspect that within a generation or two this sort of objective and constructive discussion will happen between mates with great frequency and facility.
I don't buy the old worldview, but I'm sort of still married to it. One rational response, as you point out, would be to say, "Honey, I know you disagree, but we'll
both be happier in the long run if we get divorced." Humility is not one of my strengths, but such a unilateral declaration would stretch even my ample hubris. However if I could help her gain enough of an appreciation of the new worldview that it wouldn't be so hurtful, maybe something like that will eventually be a possibility.
Yes, I obviously have some selfish motives. There's nothing but my sense of obligation and guilt to keep me from announcing, "Hey, I'm going to start looking for other partners under the following guidelines. We can discuss reasonable accommodations for your concerns, or you can divorce me if you just can't deal with it." I guess that's sort of what I do when I take up new hobbies, although none of them cut to the heart of the old marriage contract and worldview like polyamory, so there has never been the need for an ultimatum like, "If you can't deal with me pursuing photography ... well, I'll do my part to keep the divorce as amicable as possible."
Yes, my wife may not think of herself as
unhappy at present. Certainly she admits (and not infrequently enumerates) things she would change about me and our situation if she could that would make her
happier. But to suggest she could be happier by involving another person who brings those things to the table: in her worldview the only way that happens is through divorce, and while sometimes the best of bad options, that is always considered a tragic failure, and woe unto the person who initiates it for anything less than the most dire of circumstances! (In the old worldview any deviation from the boundaries of strict social institutions is considered not only purely selfish but also guaranteed to reduce the long-term welfare of everyone it touches.)
To summarize my responses to your points sequentially:
- Yes, it's my sense of obligation and guilt that binds me to the traditional marriage.
- I could keep my legal commitments in a divorce, but knowing her worldview I would feel like a terrible person slapping my wife with a unilateral divorce, even personally believing that she can do better in the long-run.
- True, old-school marriage is not a typical contract: The only exit clause is death.
- Absent the aforementioned constraints, I would absolutely give up my current arrangement to poly date.
- She's reasonably happy, but neither of us doubts she could, in theory, be happier.
- Yes, the rational thing is to say, "Here's who we are as people now. You should find a better mono/mono partner, and I should try my luck as a polyamorist. We can still be friends."