It certainly is a bad phrase to say or tell someone. Saying it can get you punched in the face if you say it to the wrong person. Why is it bad? I do not know. You are actually telling someone to do something they most probably would like.

I think fuck has bad connotations because of puritanical prudish attitudes. Somehow when someone says, “Fuck You!” it brings to my mind a form of rape. A forced fuck is what the suggestion is telling you and no one likes to be forcibly fucked.

Of course I would much rather have an angry person say to me “Fuck You!” rather than reaching for and pulling out a gun. I have had a gun pointed at me several times and I would much rather be told to get fucked a thousand zillion times than have a gun pointed at me. Here in the good old USA both are a reality.