I agree with you partially here: there are folks born that way. My readings on the subject indicated that a 10% number was thrown out for years, primarily by the gay/lesbian community. However, a summary of a study I read indicated the consensus was that among mammals including humans, the rate is really less than 5%.
Then last year a study comes out that almost 17% of teenage girls identify as lesbian (they're not, just identifying that way). If that is true, and frankly based on what I see and read I would not be surprised at all by that number, what exactly would you classify as a small portion? And how would you explain that, suddenly in a decade or so, have we started producing almost 1 in 6 lesbian females in the United States?