Look at it this way. If I were to say "Well, poop on you, Ralphfan!" Would you take offense to that? Probably not. "Screw you" and "F*ck you" are pretty much a more vulgar way of saying that. Like I said, it's rude to say the least. The additional 'asshole', 'jackass', 'dickhead', or anything like that -- comments that are widely accepted as flames -- morphs that comment into a flame.
I'm going to put this very simply.
"F*ck you". Yeah, that's really rude, but it's not a flame. It's just rude. It's no different than saying "Shut the f*ck up", to be honest.
"F*ck you, asshole" is where it becomes a flame, because at that point, you are being called an asshole, which is a flame.