Kinsey used interviews, and there are some issues with sampling bias with his work - Almost all research on this since has used surveys. Which give wildly differing percentages depending on how the question is asked. asking about behavior in very carefully neutral tones:

"How long has it been since you have had sex with a man ?: Day, week, month, year, never" "How long has it been since you have had sex with a woman? Day, week, month, year, never"

Usually gives results in the 10% range, Asking outright if people are gay or lesbian, 2 to 3 %., A lot of this can be ascribed to the fact that LBGT is not just a preference, it is also a subculture - Just because you sleep with members of the same sex does not guarantee that you feel like a part of that subculture.

Snape... That is both the author and the character being funny.