It makes sense, as Josh Barro explains in a must-read:

Even though the cost components are different, the net effect would probably be similar at the state and federal levels: positive, but very small. The Williams Institute at UCLA School of Law has reached this conclusion in fiscal analyses across various states; for example, they estimated in 2009 that legal same-sex marriage in Maine would generate $8 million in annual fiscal benefits for the state, mostly through reductions in Medicaid and other public assistance payments.

The fiscal benefits aren’t a crucial reason to support same-sex marriage, but they do lend support to one of the “conservative” cases for it. Marriage is a structure through which people depend on each other, so they don’t have to depend on the government. For gay men and lesbians to take advantage of that fiscally friendly option, the government has to make it legal for us to marry.