I can see how the typical fool enamored with current popular culture (TV, movies, music) could arrive at that guess.
Except even in pro-gay movies and TV shows, there aren’t many gays. For example, the wildly pro-gay Glee TV show has 3 known gay guys, one lesbian, and one bisexual female.
Of course, we assume that a lot more of the all-guys school was gay, but when the gay Kurt character suggested as much, his Gay boyfriend Blaine said most were straight.
So even in that environment, it’s like 1% of the population.
People just assume there’s lots of gays, and then do a terrible job of translating to percentages. I bet most people couldn’t name 10 gay people they personally know, but would say they have a “lot” of gay acquantances; even though they could probably name a hundred people they “knew”, which would make it at most about 10%.