RiskyChris said:
Perhaps a better way of putting it would be, "Why are there such sharp differences in sexual orientation to begin with?"
I'm not super-educated on the subject, and there's a good chance I'll get absolutely excoriated for this post, but hopefully, I'll be clear enough about what I mean.
In many respects, sexual orientation and fetish are internalized early on, and no ready explanation exists. There isn't a single root genetic or environmental cause.
What makes homosexuality, heterosexuality, bisexuality, etc. acceptable is that these orientations and desires don't harm anyone when properly enacted in human society. Two adults with full knowledge of the consequences consent, and all is right with the world. No one is harmed or taken advantage of. This thought could be extended to most fetishes, which can be expressed in a consensual manner.
These things are separate from the aforementioned zoophilia/pedophilia/etc. because those sexual kinks can only be fully expressed through non-consensual action that harms the other party. As a result, they are classified as mental illness.
While these two broad categories are different in how they are perceived by society (and with good reason), if we want to understand mental causes, we can't dismiss the mental links between them. Just as I don't know or didn't choose why I'm attracted to women, I doubt all pedophiles, for instance, chose to be sexually aroused by children to the exclusion of more developed individuals. And just as this "gay-straight conversion" therapy creates an enormous mental toll on the patient, forced to doubt and suppress his instinctual desires, the same is likely true for those whose sexual desires are classified as mental illness. The latter is morally permissible (unlike the former) because action on that desire hurts others, so that mental cost is a price society is willing to pay.
I'm not entirely sure where I'm going with this (since I'm not entirely sure what RobertM is driving at), but I suspect this is the line of thought he was trying to explain, and I see where he's coming from. The reasons behind why sexual preference turns out the way it does in a given individual aren't widely known, whether they are what society considers "normal" or whether they are "deviations" from that norm. If we are interested in root causes, my first thought would be that all forms of attraction might have something to teach, whether those attractions are socially and morally acceptable or not.
If I am mistaken in believing that sexual orientation and sexual fetish (~~~philia) are at all similar, please correct me. I don't intend to offend.