He pointed to a 2010 study by Columbia University researchers that surveyed nearly 35,000 Americans for about five years. During that time, some U.S. states instituted bans against gay marriage.
The study found that after those bans took effect, rates of anxiety, depression and alcohol use increased among gay, lesbian and bisexual residents. Generalized anxiety disorder -- which refers to chronic worry and tension -- more than doubled in prevalence.
In contrast, psychiatric disorders either held steady or rose a much smaller degree among heterosexuals living in those states.
"I think that's the most compelling evidence we have that there are negative health effects," Mustanski said.
As for why a ban on gay marriage would affect people's mental well-being, Mustanski thinks such laws "send a message that you're a second-class citizen."
"We know that stigma is bad for people," he said. "That's something I think any human being can identify with."
A number of medical groups, including the American Psychological Association and the American Medical Association, have said that barring same-sex couples from civil marriage has negative health effects. And earlier this year, the American Academy of Pediatrics (AAP) threw its support behind gay marriage, saying it's in the best interests of children of same-sex couples.
Not everyone agrees with the endorsements of gay marriage.
Loren Marks, a professor at Louisiana State University, published a review last year in Social Science Research that criticized much of the evidence base for the AAP and other groups' stance on children's well-being. It said that of 59 widely cited studies, almost half looked only at children raised by same-sex couples and had no "heterosexual comparison group." And in many other studies, researchers compared same-sex couples' children with those raised by single mothers.
Ryan Anderson, a fellow at the conservative think-tank the Heritage Foundation, pointed to Marks' findings as one example of the flaws in research looking at children of same-sex parents.
"The science on this is still in its infancy," Anderson said, adding that it's "premature" for scientific groups to support gay marriage based on that research.
"What we do know from years of social science data," Anderson said, "is that children tend to do best when they're raised by their two biological parents -- not just two parents."
He added that while research on single mothers gives some idea of how children fare with no father, there's little data on "what it's like to grow up without a mom."
As for whether marriage, itself, has benefits for gay adults' well-being, the jury is still out, Wight acknowledged. Studies have looked at heterosexual married couples for decades, but there is relatively little data on same-sex couples.
Based on what's known from research on heterosexuals, both married couples and lifelong singles tend to fare best as far as mental and physical health, according to Wight. "It's the people who are divorced, separated or widowed who are worst off," he said.
Of course, those are generalizations, Wight stressed. Someone stuck in a bad marriage may well be worse off than someone who got out of a bad marriage.
But whether marriage is a health boon or not, Wight said that at least having the option to marry could be good for gay and lesbian Americans' well-being. "If there are laws that are actually harmful to mental health, as the evidence suggests, then that's a public health issue," he said.
The U.S. Centers for Disease Control and Prevention has more on gay, lesbian, bisexual and transgender health.
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