Being gay is a fetish.

[One Above All]

A cure?

I really hope you're not being serious. If you're not then, cool

What is wrong with developing a treatment which switches ones orientation?

The problem is the terminology used (for there to be a "treatment" or "cure", there must be something to "treat" or "cure"), and no doubt homophobes will attempt to push it onto non-heterosexuals (once they take it themselves, of course - we all know how that story goes). Think X-Men 3, only more fabulous.
Sexuality is not a disease, and so it doesn't need a cure. However, if switching from non-heterosexuality to heterosexuality were possible and available, and if non-heterosexuals weren't being pushed to acquire it (social pressures and such) or tricked by charlatans (the so-called "therapy" for instance), I see little (if any) reason why it's wrong.

FYI: I say "non-heterosexuality" and "non-heterosexuals" because, all-too-often, people (including people within the LGBT) forget about bisexuality and asexuality. We're real too.