Republicans are much closer to reality, at 20.2%, but still far off. (The actual percentage is more like 3.5.)
What accounts for this discrepancy? Perhaps Democrats are more likely to live in places where there are larger concentrations of openly gay people. Perhaps those with liberal views are more likely to hear the news that people they know are gay. Maybe Dems are dumber. The poll does also show that less educated people have the highest estimate. Or maybe it's that women tend to be Democrats and women estimate high (29.7% to men's 19.4%). Why would women estimate high? It might be that they are more likely to key into talk about private lives, and thus hear more about various people being gay, and then they extrapolate a ridiculous number. Younger people estimate high too, and younger people tend to be more liberal.
Whatever. Why are people so ill-informed? It's embarrassing. Would people be more likely to support equal rights for gay people if they knew how small the number is? But of course, we don't really know exactly what the number is. We're all only estimating.