Interesting title no? Some authors would be scared to face such questions for fear that their theories and beliefs would fall apart under the light of science and scrunity, but I don't for the reasons I will hit right after my theory about why some gay and straight folk apparently have switched orientation, based on observations of various scientists (may provide citations later as time and refound):
Theory: Each of us are genetically born with the code for either gay, straight and bisexual orientation or all in the same way you can be born with genes for black and blond hair (Bb), only black (BB) or only blond (bb) or even a co-dominant. What is expressed depends on the dominance of the genes involved and just like any other genes (e.g. genetic diseases) having it there in your code does not necessarily mean the gene activates and it has been found genes can even be switched on and off by unknown enviromental factors, however scientists have not found a reliable mechanism to do so to my knowledge.
Also, more importantly, if you're expressing both gay and straight orientations and are bisexual then you could very well choose to act on only one and not the other and consider it choice, however that does not mean that someone with only gay genes (or only gay genes expressed) could make themselves attracted to the other sex any more than they could tell their body to grow blond hair instead of black. Same with truly straight folk. If you're straight can you make yourself attracted to the same sex or even an opposite sex person you don't find attractive? (But the latter is by far easier I imagine)
Consider also the fact that identical twins where one is gay the other can be straight, seems to say it's enviromental mainly no? However consider now the fact that between twins if one is gay then roughly 52% chance the other is gay, but between non-identical siblings the chance is roughly 22%, and between unrelated individuals 11% chance.
That would highly suggest something genetic is definitely involved as well as something enviromental that is not related to family upbringing (the enviroment would seem near identical especially in the case of identical twins).
And that where I'm at currently in terms of understanding science and the facts and I encourage others who have more technical knowledge to share with us.
As for why I am eager to humbly but confidently face things that would seem to contradict things I have stated:
None of us can claim a perfect and full understanding of God or science (that would be arrogance) so we have to always be ready to be corrected and update our understanding but we have to work with what we have. We just SHOULD remember that science cannot really contradict God, being by definition of what God is - creator of everything including science, although certain scientific facts and discoveries can seem just like certain bad events in life in the sense that they may seem to deny God's existence on the surface. Delve deeper and open your mind to the possibilities and you'll be overjoyed at the beauty of God's work and scientific rules for this universe - much like Albert Einstein was.
Those who try to use science to deny God are simply pushing they're own beliefs (and in a sense religion). Science currently cannot conclusively prove or deny the existence of God and I am not claiming that to be evidence that God exists, but assuming he does I think God purposely intends that inconclusiveness by empirical evidence and thus making the choice to have faith up to us. He's given us freedom of choice and didn't make us machines, would it make sense for Him to force belief? It is more good to be forced to be good or choose to be good? Is it not more a sign of love for someone you give freedom to to still choose you or for you to force them to be with you?
We all are free to have a belief either way, but be aware that those who try to spread atheism, mock belief in God, promote the denial of God and seek to ban all forms of expression of belief in God including prayer, art and symbols are falsely claiming their motivation is scientific. It is another religious belief, the faith in the non-existence of God.
I have no problem with scientists or anyone having a belief either way, what I do have a problem with is people who try take away freedom of belief (especially when science itself cannot make a conclusion about God), mock, ridicule, and hate others who believe differently from them or because they are different from them. That applies to Atheist, Christians, Muslims, Jews, Blacks, Whites, Asians, Homosexuals, Heterosexuals - anyone who does this, for it is HATE.
Citation (or links to citations):
Rate of homosexuality between identical twins, non-identical siblings, non-related individuals.
(my original source was from a university book but this will have to do)