Something Puzzling

Why do we say: "Go fuck yourself!", when we are angry at someone? (Interpret the 'we' here as a general, most of us, kind of 'we'.) And why is giving good head a very bad insult on most blog comment lines?
And why is someone a fucking idiot?

I mean, after all we are telling the person to do something very enjoyable in most of these cases. What is it about sexuality that makes us use one of the few things that is also called 'heaven on earth' as the worst insult we can hurl at somebody? I have never really understood this.

The reason may lie deep in the Calvinistic underbelly of America, but not everybody has roots in Calvinism. Or is the reason something to do with a combination of yearning and loathing in sex? A desire to be free of the need for the 'other', which seems to have inordinate powers of attraction? Or perhaps simple misogyny, combined with the assumed hierarchy of power in sex, with the man on top?

Why not wish someone a long and loveless life instead? Wouldn't that be an insult we could all wholeheartedly support?