In 1998 a Gallup/USA Today survey asked about whether "some practices . . . will be commonplace in the year 2025, or not?" Then they asked "do you think that in 2025 each one will generally be legal or not legal in the United States." One of the practices was gay marriage: 74% thought it would be commonplace and 69% that it would be legal. In September 2004 a Los Angeles Times survey asked, "Regardless of your opinion about same-sex marriage, do you think legal recognition of it is inevitable, or not?" 59% said it was inevitable, 31% that it wasn't, and 10% didn't know.
The sense of inevitability wasn't because most people favored same-sex marriage. The immediately preceding question in the LA Times survey asked people about a constitutional amendment that would "prevent states from legally recognizing same-sex marriage": 51% said they would favor it, with 43% opposed. It also doesn't seem to be because people assume all laws will become less restrictive. When the 1998 survey asked about "illicit drug use, such as marijuana and cocaine," 64% said it would be commonplace, but only 30% said it would be legal.