No, according to a new IZA discussion paper by Madeline Zavodny:
Controlling for observable characteristics, cohabiting gay men do not earn significantly more than other gay men or more than unmarried heterosexual men.
But she also finds that:
Cohabiting heterosexual men also do not earn more than non-cohabiting heterosexual men.
Using US data Zavodny estimates that the marriage premium, which accrues only among heterosexuals, is about 18%; this is in line with previous estimates in cross-sectional data.
Why does marriage matter? One possible answer, proposed by Becker, is that it encourages specialisation within couples. But the authors findings seem inconsistent with that hypothesis:
The results indicate a positive relationship between a man’s earnings and the education of his spouse/partner. This is similar to previous research and consistent with both a positive productivity effect and positive assortative mating.
...spouses/partners reduce their own labor supply if their husband/partner earns more. Controlling for this effect by using predicted hours yields results that are more consistent with positive assortative mating than with specialization for all three types of couples.
These results appear consistent with a recent paper by Alison Booth and Jeff Frank of academics and administrators at British universities: Marriage, Partnership, Cohabitation and Sexual Orientation: What Males Gain a Wage Premium? (PDF):
We find a statistically significant male marriage premium, an insignificant positive effect of heterosexual unmarried partnership, and no partnership return to male homosexuals. This suggests that selection may play a limited role in the marriage premium. We also provide results on cohabiting versus non-cohabiting partners, and on the academic versus administrative side of universities.
Tyler Cowen from Marginal Revolution comments:
Data on cohabitation suggest that the answer is no, whether for gay men or cohabiting heterosexuals. The standard selection story is that women are more likely to choose the high earning men and marry them. But why don't women live with these men too? Does living together not transfer enough resources? Could it be that real legal marriage is proxying for the ability to commit, which is positively correlated which other determinants of job success?
See his post for comments.