Most scientists researching this question are seeing their research pointing to at least a biological if not genetic origin of sexual orientation. This evidence is increasingly showing that sexual orientation, whether gay, straight, or bisexual, is probably set very early on--if not in the womb or at conception. In other words, sexual orientation is inborn.
Now, many people--particularly people who are opposed to homosexuality on especially religious grounds--reject this theory, saying that there are a lot of examples of people who were gay who have been "cured" and "made straight" through various kinds of "therapy," whether psychological or religious. However, it is important to remember that the American Psychological Association (APA, the national organization of professional psychologists) does not take up when or how sexual orientation is determined but it does say that there is nothing inherently wrong or unhealthy with non-straight sexual orientations; the APA further considers therapy meant to "cure" homosexuality to be at most ineffective and at worst extremely harmful to the health of people who have same-sex attraction or are gay. This kind of therapy is part of what is called the "ex-gay" movement, but it is important to note that there have been scandals in recent years of supposedly "cured" gay people being caught in gay bars or engaging in gay relationships, and the largest ex-gay organization in Great Britain actually disbanded a few years ago because they realized they were having no success in curing people--slowly coming to the realization that sexual orientation cannot be changed.
Now, people who believe that sexual orientation can be changed are usually confusing homosexuality and heterosexuality with bisexuality; obviously someone who is bisexual can be sexually attracted to people of either sex, or conditioned or taught to seek out relationships or sexual experiences with only the opposite sex. However, truly heterosexual and truly homosexual people cannot truly change their sexual orientation; at best, they can become celibate (avoiding sex), which is not the same thing as changing one's inborn sexual orientation.
More and more research will be done in this area in the next decade, and it is likely that one or more biological or genetic sources of sexual orientation will be found. However, it is safe to say that sexual orientation is at least inborn and not something chosen or mutable. It's simply the way someone is born. WikiAnswers.com