Has been explained already. There is cursing and there is the use of blunt or dirty language to describe something, e.g. an action or a body part. "Shove it up your ass" on Lemon is not to be taken literally (you actually can't shove your future up your ass), while "fück you when she's done" or "I can barely fit my dick in my pants" are. It's easy (and often unimaginative) to say "fück", everybody can do that, but it takes some skills to paraphrase "fück" in a clever, interesting way.
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