Perhaps many of the former gays went straight because its easier to live with that lifestyle.
The whole issue is a bizarre one for me.I never felt the slightest inkling of attraction to to other guys.Yet I always got along better with men in social situations.I felt I could be myself with other males and always felt awkward around girls and women,like I was putting on an act.
Yet I always had a strong sex attraction to women.Which led to many very dysfunctional relationships with them since it seemed we were replaying the old Men for Mars,Women from Venus paradigm over and over.
Perhaps that's true of some ex-gays. I don't think that can be said of the 5 ex-gays in the I Do Exist video. You can also read how others left the lifestyle here.
You should also read what Robert Spitzer has said on the matter. Spitzer Study Published: Evidence Found for Effectiveness of Reorientation Therapy. If you realize who Spitzer is, what he used to believe and what he believes now about homosexuals leaving the lifestyle, that should tell you something about the possibility of change.