Up until reading some of the explanations for them, I was too quite weirded out by the thought. I still am to an extent. Especially toward the ones that would include anything sexual, I don't understand why someone would want to see something like that.
I see nothing wrong with a drawing of you, with animal-like qualities, or even you as the animal of choice with minimal of your own features. But, I find it highly creepy and weird to hear of people that want to see a picture of an animal or multiple doing sexual acts, whether or not they are an actual animal or one of these 'fursona' ones or even people dressing up as animals for sexual relations.
Though I don't think it's necessarily prejudice to refuse doing such art on any scale, as I don't for one second find it unreasonable to refuse to make any kind of art for anyone that you personally do not feel comfortable with, it's your prerogative.
Most of the thing people stated before me is just about what I was gonna say. But yeah, there are those occasional... I'm sorry, but "weirdos", who take it too far and it becomes their fetish or something.
I was under the impression that it was an identity... as in, that's the creature the uhh, owner(?) actually is, while they're stuck in a human body. or, in its most basic form, is it just like an avatar?