The tax code does not allow you to deduct cost of living expenses, as a general principle. So someone making $200k in Iowa has more disposable income than someone making $200k in NYC, but the Fed. Gov. taxes them the same. To me, it follows that the tax code shouldn't impose an extra burden because of an inference about your living expenses made from your marital status.
Why does it have to follow in that direction? Maybe because of the marriage penalty it follows that the tax code should take into account cost of living based on your zip code.
You've said several times that the tax code is set up to tax increases in wealth.
That's a fair point too, but I'm taking the premise of the current tax code (that cost of living doesn't matter), as a given. I'd be less unhappy with the marriage penalty if the tax code otherwise accounted for cost of living.