Actually, slightly less than 4%.
"...they think that 23 percent of Americans, or almost one in four, are are gays or lesbians, a Gallup survey released Thursday revealed. That's way off: The polling organization most recently found that less than 4 percent self-identify as lesbian, gay, bisexual or transgender."
http://www.bloomberg.com/politics/articles/2015-05-22/americans-vastly-overestimate-size-of-lgbt-population