I note an earlier post states that that sexuality has changed "both ways" (gay to straight and straight to gay?). I'm not sure of the proportions here who are changing - 1%, 5%, 10%, or even 50%). But whatever the proportion is -innateness and change is not mutually exclusive!?
And what does "change" mean. A realisation (cf the late George Melly) that after a period of indulgence with one gender that you prefer the other gender (or perhaps both). Or pyschological counselling; or chemical treatment? Electric shock treatment?