Earlier quoted context omitted.
The US has joint filing for married couples so that's not the case, whereas it would be in e.g. the UK.
It most certainly is the case in the US... http://en.wikipedia.org/wiki/Marriage_penalty
AKA 2 incomes of $87,850 x2 = 175 700 vs extra 879$ in taxes.
At extreme incomes of say 20+M a year or below 18k it's actually far less than 1/2%.
However, there are huge benefits for SS for being a married couple even more so if there is a large income disparity, so on average if they plan ahead Marriage tends to lower taxes. On top of lower expenses which enable better tax avoidance mechanisms.