There are still tax disadvantages for some married couples (used to be all, assuming both worked).
However, there are also plenty of welfare programs that are means tested so marriage is ruled out.
(One of the leading theories for the number of babies born out of wedlock rising so much since the early 1960s is the end of the shotgun wedding: the baby daddy counters by pointing out that the mom will lose out on a lot of benefits if she gets married)
There is an additional Medicare tax of 0.9% if single income exceeds 200k, but if you file jointly, that limit is 250k. So a married couple with each earning 125k+ would pay more tax than an unmarried couple where person made 200k.
This hardly mattered years ago, but with median wages getting around 70k+, a lot more people pay this penalty for getting married.
However, there are also plenty of welfare programs that are means tested so marriage is ruled out.
(One of the leading theories for the number of babies born out of wedlock rising so much since the early 1960s is the end of the shotgun wedding: the baby daddy counters by pointing out that the mom will lose out on a lot of benefits if she gets married)