I’ve been looking up some studies on the subject (bit of a busman’s holiday for me since I’m a librarian 
) and there is some evidence that some people can to some extent change their sexual preferences. The following isn’t a direct response to you, Mangetout, I’m just taking this as an opportunity to share with the group.
There was a controversial 2003 study by Robert L. Spitzer* that involved interviewing people who self-identified as successful conversion cases. Spitzer included only those who reported having sustained some change in their orientation for at least five years. Many of the participants did report dramatically reduced levels of same-sex attraction since undergoing some form of therapy to change their sexuality, and all but one of the participants had completely given up homosexual sex. When asked if they were ever “bothered by unwanted homosexual feelings”, 26% of the men and 49% of the women chose “Not at all”.
Of course, this means that 74% of the men and 51% of the women were at least sometimes still “bothered by unwanted homosexual feelings”. 45% of the men and 18% of the women also reported that at least 20% of the time they still fantasized about members of the same sex while masturbating. Taking this study at face value (it has been criticized for methodological problems) there do certainly seem to be people who have made voluntary changes in their sexual preferences, but few who managed to completely abolish all homosexual desire. And remember, this wasn’t a study of people who’d simply undergone “ex-gay” therapy, but only of those who self-identified as having made a sustained and significant change in their orientation.
Ariel Shidlo and Michael Schroeder’s 2002 study “Changing Sexual Orientation: A Consumers’ Report”** seems to be the best study on how successful “ex-gay” therapies actually are. This study involved interviewing 202 people who had undergone sexual orientation conversion intervention. 13% (26 people) of these people considered themselves to be success stories, but nearly half of these still experienced repeated “slips” into homosexual behavior. Of the remaining 14 people, 6 reported having successfully stopped engaging in homosexual behavior but still experienced some same-sex desire and did not self-identify as heterosexual. Only 8 people (4%) self-identified as heterosexual and reported exclusively engaging in heterosexual behavior with little or no same-sex desire.
Interestingly, 7 of these 8 people worked as “ex-gay” counselors, 3 on a volunteer basis and 4 for pay. In fairness, people who were truly successful in changing their sexual orientations would likely have an interest in helping others who wanted to do the same. But I can’t help but think that people who are themselves “ex-gay” counselors would have good reason to exaggerate how successful their own conversion has been.
The majority (77%) of the participants in Shidlo and Schroeder’s study indicated not only that the therapy hadn’t changed their sexual orientation, but that it had done them significant long-term damage. (Another 10% reported experiencing neither change in orientation nor any long-term psychological harm.) 11 participants reported attempting suicide after undergoing “ex-gay” therapy. Only 3 of these had previously attempted suicide. So it looks to me like “ex-gay” therapy is at least as likely to make people want to kill themselves as it is to result in a successful conversion to heterosexuality.
Based on this I continue to believe that while it’s possible that some very small minority of people may be able to genuinely change their sexual orientation through voluntary effort, the vast majority cannot and for the sake of their own mental health shouldn’t even try.
*There are some people who are pushed into “ex-gay” programs, sometimes teenagers forced into it by their own families, but there are also people who genuinely wish to change their sexual orientation because they believe that it is disordered, that it is sinful, or they are otherwise unhappy about not being heterosexual. What’s truly tragic is that the majority of these people will wind up even more unhappy for having tried to change their orientation.
*“Can Some Gay Men and Lesbians Change Their Sexual Orientation? 200 Participants Reporting a Change from Homosexual to Heterosexual Orientation”, Archives of Sexual Behavior, Vol. 32, No. 5, October 2003, pp. 403–417.
**Professional Psychology: Research and Practice, 2002, Vol. 33, No. 3, 249–259