[anamax]

A marriage differential is a mathematical consequence of progressive marginal rates and community property (specifically the ability to shift income between spouses).

To put it another way, eliminating the marriage differential[1] in all cases requires giving up progressive marginal rates or community property.

Which one do you want to give up?

[1] The current US brackets and deductions taxes some married couples more than comparable pairs of individuals, taxes some less, and is basically a wash in other cases. It's easy to move couples between the first two groups and you can move some of each to the third, you can't move all of them.