Ygleias is being too clever by a half:
While what he says is true, what he misses are the nonmonetary benefits of marriage, or perhaps even having a roommate. Sure, you can split the cost of living, but the mutual support goes much further. Childcare duties (unpaid!) can be split – and children better cared for. When one partner is ill, the other can pick up the slack, drive them to appointments etc, ensuring a faster recovery. Bouts of unemployment do not lead to nearly as dramatic a loss in income.
The above neglects the emotional benefits of marriage and family stability, which of course affect pecuniary affairs as well.