Not very, according to the CDC:
The National Health Interview Survey, which is the government’s premier tool for annually assessing Americans’ health and behaviors, found that 1.6 percent of adults self-identify as gay or lesbian, and 0.7 percent consider themselves bisexual. The overwhelming majority of adults, 96.6 percent, labeled themselves as straight in the 2013 survey. An additional 1.1 percent declined to answer, responded “I don’t know the answer,” or said they were “something else.”
The figures offered a slightly smaller assessment of the size of the gay, lesbian and bisexual population than other surveys, which have pegged the overall proportion at closer to 3.5 or 4 percent. In particular, the estimate for bisexuals was lower than in some other surveys.
Eugene Volokh notes that, according to the CDC, lesbian and bisexual women slightly outnumber their male counterparts:
1.8 percent of men self-identify as gay and 0.4 percent as bisexual, and 1.5 percent of women self-identify as lesbian and 0.9 percent as bisexual. The results are generally in the same ballpark as past estimates — and far below the long-debunked 10 percent estimate. But past data that I’ve seen had suggested that there were about twice as many gay or bisexual men as lesbian or bisexual women; this data suggests that there is no such gender gap.
Meanwhile, Arit John considers the importance of framing:
The survey comes up with a number that’s lower than the 3.5 to 4 percent figure found in other surveys. And as we’ve seen from past surveys, what’s asked matters. Specifically, the broadness of the answers available to respondents makes a difference.
In 2007, researchers at Cornell University interviewed 20,000 individuals in 80 communities. “Mostly heterosexual” was an option for respondents, and the results showed a higher percentage of nonheterosexuality, especially among women:
85.1 percent of the young women identified as heterosexual; 0.5 percent reported no sexual identity; and the remaining 14.4 percent were sexual but not strictly heterosexual, i.e. either lesbian or bisexual. Among young men, 94.0 percent identified themselves as heterosexual; 0.4 percent of the men reported no sexual identity; and the remaining 5.6 percent identified as gay or bisexual.
Meanwhile, the study showed that gay and lesbian Americans were healthier than US heterosexuals in some respects and less so in others – more likely to drink, for example, but also more likely to exercise.