I had the displeasure of proofreading someone's dissertation on homosexuality. The original, unfounded claim, which the author tracked down & quoted from some gay lobbyist, was that there are only 10% exclusively heterosexual students, only 10% exclusively homosexual students, and that most people fall in a continuum in the middle... then the media spun that to 10% homosexuals. That, of course, is about as silly as the "a homeless person dies every 3 seconds" claim.
Sounds like it was lifted straight from Kinsey - IIRC, he had a seven point "scale", ranging from "exclusively heterosexual" on one end to "exclusively homosexual" on the other, with gradations in between.