It's a generally around a 1/2 a % penalty which is swamped by other benefits.
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.