One reason behind the surge is evident from both opinion polls and interviews for this article: there is a marked increase in the number of people of all ages who know gay men and lesbians.

Put another way, when it comes to public support of same-sex marriage, familiarity breeds contentment.

In May, 69 percent of adults in a New York Times/CBS News poll said they had a gay colleague, close friend or relative, compared with 44 percent in 2003. By age group, 77 percent of baby boomers in 2012 knew someone who was gay, versus 42 percent in 2003. For 18- to 49-year-olds, it was 70 percent, up from 49 percent; and for seniors, 54 percent, up from 27 percent.

In 1988, 15 percent of 18- to 34-year-olds — the age of baby boomers at the time — supported same-sex marriage, according to the General Social Survey, which is financed by the National Science Foundation. Now, among 18- to 34-year-olds — the millennials — 63 percent do.