And good for society. When one of the premier medical journals in the world says it, it must be so.
What they are actually saying is that marriage in general is good for people. Several studies have shown that married people tend to be healthier and take better care of themselves and each other. They are more likely to see doctors regularly, so they get preventive care which is cheaper for society than waiting to treat later stage disease. Marriage also facilitates care since it is legally easier for a physician to discuss things with a legal spouse than a partner whose legal standing may be somewhat nebulous, especially compared to blood relatives. It also clears up ambiguity of who might be responsible for children. There is no evidence at all that gender or orientation decreases that benefit.
I have decried the politicization of science forever, but I can definitely get behind the scientificization of politics.