It might be helpful to explain how all this developed historically. After all, Canada is similar in many ways, but there is no “marriage penalty” (except in Quebec, which bases many deductions on total family income).
Historically, there were no income taxes until WWI. Even then, the ordinary working zhlub didn’t pay them–they were for rich people. And there were no joint returns and no income splitting. Except in California and a couple other states whose basic laws were based on Spanish civil law (which may have been based on the old Roman code). In those states, income was always considered to be joint, half going to each spouse. This produced a basic unfairness. Taxes were lower if you lived in one of these states. Income splitting was introduced to level the field. (During WWII, taxes were raised enormously, the top bracket hitting 91%.)
This worked well for the then-typical single income couple since they paid a lower overall tax. But then another unfairness arose. If the spouses filed separtely and one spouse took the standard deduction and the other one itemized deductions, using, for example, mortgage interest as a major item, then they were indeed taking advantage of the system. So lower standard deductions were introduced for separate filers. Now it is unfair in the other direction, especially when the spouses have roughly the same income, and that is what the fuss is about.
Canada avoided some of this by never having income splitting, never having a standard deduction, and never allowing deductibility of mortgage interest.
Both systems still have substantial built-in subsidies for home-owners as opposed to renters. In Canada, you have to have equity before you get the subsidy. For example, the house my wife and I occupy has a market value of over a half million dollars. That represents a genuine investment and we pay no taxes on the proceeds of that investment. If, for example, I traded houses with our next-door neighbor I would suddenly acquire a rental income of, say, $40,000 without a single change in my lifestyle and if you think that is not a subsidy, you are not thinking it through. A friend of mine recently sold his house and moved to a condo. He will pay taxes on what he earns by investing the difference. but had moved to a rental apartment, he would have to pay taxes on the total investment income, not half of it.
But I digress. The point is that the tax system is inherently unfair but the reasons are largely historical and explained by how we got there. If I were dictator, I am sure I could create a much fairer system and it would certainly not build in a marriage penalty.