I'd like to post my response here, since it struck me as a good summary should anyone ask me the same question again.
In evaluating ex-gay testimonies, I’d point to three things.To this I might add that even the most impressive ex-gay testimonies I have heard turn out to be more complicated once you scratch beneath the surface. A common report is that attractions resurface and temptations still occur when the subject is feeling down, stressed or weak. That suggests to me that they are managing their attractions, but their basic orientation towards males remains unchanged.
First, my own experience. I was a Pentecostal and I fought “same-sex attraction.” I can testify from my own experience that the capacity to con yourself into thinking you have overcome or are overcoming your basic sexual orientation is huge. I was in denial a long time, knowing deep down that I was still basically attracted to men. There were times when I was so “victorious” in the Christian life that I thought infrequently enough about men that I could convince myself I’d changed or was changing. I tried desperately to exploit the 20 percent of me that was attracted to women (yeah, there’s a hint of bisexuality in me). For periods I could “triumph,” but it never lasted. Nothing fundamentally changed.
Second, other people’s experience. In the whole ex-gay movement, the examples of people who claimed to be “healed” of homosexuality only later to turn back or be caught out are numerous. At least two founding members of Exodus, the world’s biggest ex-gay organization, left the movement and admitted they were still gay. Another of their head honchos got married, but had to leave too when he was photographed chatting up guys in a gay bar. Jeremy Marks, an Anglican who founded Courage, one of the UK’s main ex-gay ministries, did a total turnabout on the issue when he realized after years it just wasn’t working for him or anyone else. I talk to people every day who have survived the ex-gay movement, some of whom have spent thousands on therapy, counseling and ministry over the years.
Third, the scientific evidence. There basically is no scientific evidence that sexual orientation can be made to change through therapy, ministry, prayer etc. There have been two or three deeply flawed studies, but even the best of these (carried out in the US a couple years ago) points to a minuscule success rate. The consensus of psychologists and psychiatrists is that this kind of therapy is useless at best and dangerous at worst.I write regularly about the "ex-gay" phenomenon at ExGayWatch.com.
That’s not to say sexual orientation can never change, of its own accord, say, but can it be made to change? Everything I know to be true says no, it can’t.
So what do I do in the case of [our ex-gay acquaintance]? Assume he’s lying? No. There are many possible explanations. Maybe he grew naturally into heterosexuality? (As I said, sexuality changes, it just can’t be forced to change.) Maybe he was bisexual all along? Maybe he’s kidding himself? (I kidded myself a long time about my success at changing.) All I can really say is that based on my experience, others’ experience and most importantly the scientific research, that God made him straight overnight is the least likely explanation.