Everyone shits, but "shit" is still considered a bad word because of its material properties. Fuck, on the other hand, is something everyone loves doing, except when done by force... I guess the reason it's still seen as a bad word rather than a "love word" (besides old religious believes that sex is actually intrinsically bad, which some people might still hold) is that "fuck" is unconsciously associated with forced sex (rape, to be clear) while consensual sex is associated with other words/phrases like "make love", "intercourse" etc. which do not have the same bad connotation.