Get Bent

What is the deal with "fuck you"? Arguments end with this two words, sometimes they begin with them. What I don't get is men and women spend a better part of their days trying to have sex, so why do we tell someone to "get fucked" when they were probably already working toward the act anyway? Or how about that car salesmen "fucked me over"? Who doesn't want to get fucked over and often? How can you take a pleasurable thing and then offer it in a mean way?

If you are arguing don't you want to wish a misfortune like "fat you" or "get broke"?