Nearly every politician in Washington agrees that the marriage penalty is unfair. It seems unfair. But that doesn't mean anything is going to be done about it anytime soon. For one thing, there's no easy way to remedy it. A progressive income tax with multiple rates is a complicated proposition, and somebody is always going to get the short end of the stick.
Arbitrariness is in the nature of a code with brackets.
In the '50s and '60s, single people paid higher tax rates than they would have if they had been married. Now married people are complaining. But if they get their fix, it won't be long before the singles pipe up again. The second reason is class politics. Both parties agree on cutting taxes for married couples.