The American Psychological Association (APA) writes:
What causes a person to have a particular sexual orientation?It goes on to say that there is no known therapy for changing sexual orientation that has been proven safe and effective, but does not say that it is impossible or that there is evidence that it is impossible.
There is no consensus among scientists about the exact reasons that an individual develops a heterosexual, bisexual, gay, or lesbian orientation. Although much research has examined the possible genetic, hormonal, developmental, social, and cultural influences on sexual orientation, no findings have emerged that permit scientists to conclude that sexual orientation is determined by any particular factor or factors. Many think that nature and nurture both play complex roles; most people experience little or no sense of choice about their sexual orientation.
The APA has just endorsed a stronger statement, as the NY Times reports:
The American Psychological Association declared Wednesday that mental health professionals should not tell gay clients they can become straight through therapy or other treatments. ...Switching churches? Some silly shrink is going to try to tell you what church to attend? The report does not explain how well that works.
No solid evidence exists that such change is likely, says the resolution, adopted by a 125-to-4 vote. ... Instead of seeking such change, the association urged therapists to consider multiple options, which could include celibacy and switching churches, for helping clients live spiritually rewarding lives in instances where their sexual orientation and religious faith conflict.
The report does not go so far as to say that sexual orientation is immutable. It says that certain therapies are likely to fail, but most psychological therapies to change behavior fail anyway.
When someone expects me to have an answer to these questions, I point out that there are experts who have devoted their lives to them, and they do not have an answer, so how could I have answer? Any opinion you get from me or anyone else is just an opinion, as no one has proved that any answer is correct.