I think the real number is under 1%. I think that somewhere around 15 years ago, networks figured out that they can cash in on the wow factor of gay characters in their shows. It snowballed from there, and now most series have at least an occasional gay character... for ratings.
Somewhere in there, teens starting believing that just because every MTV "reality" series showcased at least 1 gay contestant, or 2 chicks making out, that the straight to gay ratio must be 5:1.
To believe that 10% of the population is gay, you'd have to believe that 10 out of every 100 people you see on a daily basis is gay. No way. Not even close, man.
The loudest and most persistent groups will always skew the public's perception of a topic, situation or count. For example, a small number of people complain about an "indecent" tv incident, and the media reports it as "massive" or an outrage. Or, maybe, the real number is 50% and I will become gay one day?