My view is that sexual orientation can be changed, but the change is not always voluntary. The evidence that I have for my view comes primarily from the instances where people, especially women, report a change in sexual orientation around mid life (age range 45 to 65 years old). Psychologists Lisa Diamond has compiled the evidence for those changes and talks about some of it in her book, Sexual Fluidity, Understanding Women's Love and Desire. Dr. Diamond comes up with a concept called 'sexual fluidity' and uses that to explain these changes.
Whenever I argue this point, some in the LGBT crowd tend to point out that sexual orientation is not chosen. But then I respond by telling them that a change does not have to be voluntary. The change can happen involuntary. It's entirely possible for someone to have an underlying or baseline sexual orientation, and then it starts to expand, perhaps from gay or straight to bisexuality. I would imagine that it's easier to add to your pre-existing sexual orientation than to try to remove your orientation and replace it with some thing else. I also don't discount that sexual orientation can be changed as a result of choice (therapy or behavior modification) if we strip it away away from religious or moral obligations. In fact, we could even do it in a positive context, like a heterosexual undergoing therapy to become bi if they were curious about experiencing same-sex relationships.
One implication of my view then is that people can potentially choose or change to any sexual orientation. Now whether or not they should gets into morality and that's a different topic.
Here's a good article that factors in both sides of the debate on this issue:
For some people, sexual orientation and desire are not rigid or continuous throughout their lives; rather, they can be fluid and change over time.
Unfortunately, what many communities share in conceptualizing sexual identity, is a belief in its fixed nature. From the “born this way” rhetoric of the mainstream gay rights movement seeking to root gay and lesbian identity as established at birth, or the search for a “gay gene” seeking to legitimate gay and lesbian identity through biology, sexual identity – more colloquially described as ‘sexuality’ – is cast as a characteristic or trait that does not change. Mainstream LGBTQ discourse subscribes to a model of identity formation and development that assumes an early discovery of same-gender attraction, a period of hiding that attraction (being “in the closet”), an explosive coming-out process by which that attraction becomes a publicly held identity, and finally a stabilizing of that identity over the long-term – typically, the remainder of an individual’s lifetime. For some people, sexual orientation and attraction are very fixed; however, this is not the case for all.
This depiction of fixed sexuality is often established in opposition to the conservative argument that sexuality is both “unnatural” and “a choice” – an argument that seeks to delegitimize same-gender relationships and orientations by virtue of them being non-normative, and by virtue of them not fitting into a prescribed perception of “appropriate” human sexuality often informed by religious beliefs. Countering this argument, then, often involves arguing that human same-gender relationships are both “natural” and “not a choice” – through examples of same-sex relationships in animals, same-sex human relationships in history, and the aforementioned framings of sexuality as a biological characteristic established at birth.
Human sexuality, however, is understood currently as more complex than either of these binary depictions typically show it to be. We now often differentiate sexual, romantic and aesthetic attractions and identities from each other, framing each as a constantly-changing characteristic shaped by past and current experiences, other held identities (whether racial, class, gender, ability, religious and/or others) and an individual’s own agentic desire. An individual may, for example, desire to have sexual relationships typically with women, but find themselves romantically attracted to people of all genders and aesthetically attracted to more androgynous forms of gender expression. Many years later, the same individual may find that their sexual, romantic and/or aesthetic attractions and identities have changed –perhaps as a result of living in a different environment and interacting with different communities, personal and/or spiritual exploration, a significant formative sexual or romantic experience, personal choice, some combination of all of these or for a different reason altogether.
The only disagreement I have with the article is that I believe people are born a certain way due to the fact that we have early childhood attractions (our baseline sexual orientation). But from there, I agree with the article in that we can expand or shift our sexual orientation; it may happen on its own as we experience life or perhaps it can happen voluntarily, as well.