Higher numbers can be obtained when asking about lifetime sexual experiences, rather than identity. The Williams Institute found that, overall, an estimated 8.2 percent of the population had engaged in some form same-sex sexual activity. Put another way, 4.7 percent of the population had wandered across the line without coming to think of themselves as either gay or bisexual. Other studies suggest those individuals are, like the bisexuals, mainly women: The same CDC study that found only 1 percent of women identify as lesbian, for example, found that 13 percent of women reported a history of some form of sexual contact with other women.
"Estimates of those who report any lifetime same-sex sexual behavior and any same-sex sexual attraction are substantially higher than estimates of those who identify as lesbian, gay, or bisexual," the Williams Institute's Gary J. Gates concluded.
These numbers are significant because identity -- and not behavior -- is the central determinant of whether or not someone will seek a same-sex marriage. A straight woman who makes out a couple of times with a female friend in college is not going to seek a same-sex marriage, nor is a guy who fooled around once with a male friend while drunk in high school. Neither individual is demographically relevant to the question of how often same-sex marriages will occur. And it's not clear at all what fraction of bisexuals will seek out same-sex marriages.
Overall, there have been fewer than 75,000 state-sanctioned same-sex marriages in the United States since they began to be permitted less than a decade ago, according to an estimate by Marriage Equality USA. Over the eight years since Massachusetts became the first state to legalize same-sex marriage in May 2004, 18,462 same-sex couples married in the Bay State. Another 18,000 were estimated to have wed in California during the few months before Proposition 8 passed in 2008, banning future ones; those marriages remain on the books, as the proposition was not retroactive. It's not totally clear how many same-sex marriages have taken place in New York, Connecticut, Vermont, New Hampshire, and the District of Columbia, the other jurisdictions where it is permitted.
Of course, gays aren't the only minority population that has an outsized place in the public imagination. Americans also "vastly overestimate the percentage of fellow residents who are foreign-born, by more than a factor of two, and the percentage who are in the country illegally, by a factor of six or seven," according to a 2012 Wall Street Journal report on the social science of estimating minority groups. In 1993, a group of political scientists reported in Public Opinion Quarterly that "The extent to which minority populations are perceived as a kind of threat is ... related to perceived proportions, though the direction of causality cannot be determined." Correcting the misimpressions about the size of a minority group hasn't been proved to have much impact on beliefs about them in the short-term, but that doesn't mean that they might never.
One thing's for sure: it's hard to imagine the fact that so many think the country is more than a quarter gay or lesbian has no impact on our public policy.