As elephino point out whether there is a tax advantage for being married depends on the circumstances.
Initially the idea of a married tax bracket was to help married men who supported a wife who didn't work.
The complaint was that a married guy paid the same taxes as a single guy but his income was divided in two.
So his tack bracket should really be at half his income (half for him, half for his wife).
It was advantageous to be single, as the married man paid the same taxes plus supported a wife.
So they tried to even it out with a married tax schedule.
But that doesn't help out so much when the husband and wife both make the same or make a lot.
There is no clear tax advantage for married people.
It depends.
Just like all of the other tax loopholes.
Some get an advantage and some lose out.