In a 2001 poll by the Pew Research Center—a nonpartisan organization that conducts public opinion polling, demographic research, and other empirical social science research—Americans opposed same-sex marriage by a margin of 57% to 35%. Polling in 2017 shows a significant change in those numbers, with 62% expressing support for same-sex marriage versus 32% opposed. This most recent poll also marked the first time that a majority of those in specific demographic groups, including Baby Boomers, Republicans and Republican-leaning independents, did not oppose same-sex marriage.
Check out the Pew Research Center site for an interactive look at the changing views on same-sex marriage.