Polling companies seldom ask questions on which people can make obvious fools of themselves, since those can raise questions about the value of opinion polls.
Looking at the demographic crosstabs, it's evident that low intelligence people were most likely to wildly overestimate the percentage of homosexuals: 53% of people making under $30,000 annually said that at least 25% of the population was gay, and 47% of those with no more than a high school education. 43% of Democrats versus 24% of Republicans got the question wildly wrong.
In general, people are terrible at estimating or remembering demographic statistics. 2001 Gallup survey, right after the release of 2000 Census results, found that the average American estimated that 33% of the population was black and 29% were Hispanic. That adds up to 62%, but who's counting? Not most people.
In that 2001 survey, nonwhites estimated that 40% of the population was black and 35% was Hispanic (adding up to 75%). In contrast, people claiming postgraduate degrees estimated that 25% were black and 24% Hispanic (only about double the Census numbers), which proves the value of advanced education.
Here, roughly, is how people think: You ask somebody how many Americans are Hispanic and they think of that guy on Saturday Night Live in the 1970s saying "Beisbol been berry berry good to me." And what's more American than baseball? And there are a lot of Latin ballplayers. But the guy on SNL who said that was black. So that means there are a lot of guys who are black comedians and also a lot of guys who are Hispanic baseball players.
Something that people almost never do is think about fields in which the group they are being asked about is rarely represented. If you ask people about gays, they think about fashion designers, musical actors, Republican politicians, interior decorators, and the like. They don't think about, say, oil company engineers or baseball players.
Four years ago on iSteve, I asked "Where are the famous old gay baseball players?"
Baseball players are extremely famous. I have a book by Bill James that lists his picks of the top 900 baseball players of all time, and I'd heard of the large majority of them, plus most of his picks for the next 225 best players. I could tell you facts about well over 500 baseball players.
I pointed out that while I had heard of two minor major league players were gay, I had never heard of a famous player who turned out to be gay. I said I'm sure I'm not aware of some, but I would suspect that no more than 1.0% of famous baseball players were homosexual.
This is not to say that baseball players are representative of the general population. I'm just saying that famous baseball players are one intensely studied group, of which very few turn out have been gay.
I'd heard lots of rumors over the years, but most of them were obvious gay fantasies about handsome, manly athletes like Mike Piazza and Sandy Koufax (The elegant and taciturn Koufax, who grew up the son of a rabbi in Brooklyn, has lived most of his post-retirement life in conservative small rural towns with his various wives and girlfriends, which would be an extremely improbable choice of locales for a gay Jewish celebrity.)
However, a number of commenters pointed to one famous old baseball player as not being publicly out of the closet, but his homosexuality being open knowledge. He's not a Hall of Famer but he's definitely one of the top 500 players of all time. Plus, during his long career, he was fairly famous for being famous. That's because he'd do the kind of socially gracious, media-friendly things that ballplayers almost never do. For example, when traded to Montreal, he learned French, which made him hugely popular with local fans.
The funny thing is that I'd never heard rumors about him, probably because he's not the kind of individual to excite gay fantasies: not a great athlete but instead a highly skilled craftsman. Instead, he's a best-case scenario for a stereotypical gay athlete: famously charming, cultivated, fastidious, and sociable. I won't put his name here, but if you are interested, you can make your guess, then go look at the comments to my 2007 post and see if your guess matches up.