Both can be true: make less money (who would want to have a gay co-worker?) and have more disposable income due to the lack of children, as you noted. But when the researcher says that the lesser income may account for worse health care, that is possible since in our system health care is paid for by employers.
It is possible, but there are a few other factors; e.g., gays are considered single for insurance purposes, and thus relatively cheap. I wonder if this income disparity would vanish if the data were corrected for age - gays tend to die younger than heterosexuals, and wealth increases with age.