> "These people are often fighting these feelings for years: weighing their natural biological impulses against higher-level rational thought. Rational thought that has been crafted from the narratives of a culture, encouraging marriage as a virtue."
Ah, but I think this concedes too much! The thrust of your argument is basically that we can boil down this very complex web of variables that make up the decision tree to seek a divorce to a cold calculation of, "Can I do better?" Were you to model this behavior in some simulation you would see people exiting and entering relationships very quickly based upon the speed of calculation.
But the actual calculation of whether to seek a divorce is not so simple. It is a complex web of interacting considerations and "rationalizations" that do battle with the impulse you would boil it all down to.
Does this impulse exist? Absolutely. Does it win out in many circumstances? Absolutely. Is every divorce ultimately a simple case of this impulse winning out over other considerations? I think that goes too far. And what about all those cases where the impulse does not win out? Would you reduce all of those to the calculation returning, "No, I don't think I could do better!?"
Regardless, the story of why people divorce is much more complicated and rich than this simple impulse to look around and see greener grass.