Dictionary.com said:
fet·ish [fet-ish, fee-tish]
–noun
1. an object regarded with awe as being the embodiment or habitation of a potent spirit or as having magical potency.
2. any object, idea, etc., eliciting unquestioning reverence, respect, or devotion: to make a fetish of high grades.
3. Psychology . any object or nongenital part of the body that causes a habitual erotic response or fixation.
Imagining a scenario where two straight characters are gay isn't a fetish. However, habitually doing so is another story, especially if you feel particularly strongly about it or it gets you sexually excited BECAUSE it's two straight characters doing things they normally wouldn't.
Personally, I think the entire thing is stupid as hell. Ffs, live and let live, people. Who gives a shit if people like yaoi because of the romance or the chance to see one guy take it up the ass? For God's sake, it's not bothering you unless they're shoving it in your face.
I also think it's stupid when people like yaoi, but find yuri disgusting, and vice versa.
Then again, I think it's stupid to get bent out of shape over things that are fucking works of fiction.
THIS MEANS YOU TOO BEX
As a footnote: it's no secret that I find dark hair to be a huge turn-on. It doesn't get me sexually excited - it's just what I like best. Yeah, it's a fetish, but it falls under the second definition of such.
Fetishes have a negative connotation and stigma, and it's absolutely fucking retarded.