From the American Consumers Newsletter:
Support for gay marriage hovers just below the 50 percent mark, according to the 2012 General Social Survey. Forty-nine percent of American aged 18 or older now agree that gays and lesbians should have the right to marry. Support for gay marriage has climbed rapidly over the past few decades. In 1988, only 12 percent of the public supported the right of gays and lesbians to marry.
One factor behind the rising support for gay marriage is generational replacement.... Changing attitudes also play an important role in the growing support for gay marriage...
A Pew Center poll shows similar tracking.