Why is the "F-bomb" such a taboo and extreme word, especially if sex is casual and pleasurable?

I learned to simply flip the bird when I was twelve. Because it was easy, and honestly the only thing some people understand. Calling someone a ‘Toad Tush” or arguing with them or just saying ‘darn it’ just doesn’t get it.

Today, the F-word is shouted by twelve year olds and gangstas alike. But I still don’t understand why ‘Fuck’ and the concepts around it are such an insult.

‘Shit’ is indeed an offensive word—but is lesser and weaker than ‘fuck’,
yet “Fuck you” in truth sounds like wishing someone a good sexual experience. And sex today is about as casual and common as watching a good movie.

Admittedly, calling someone a racist, or, covertly, calling someone a racial slur is worse than saying ‘Fuck You!”—the F-bomb, really just meaning sex, is top of the pile. Why is that?