George Carlin became quite famous for his English-language words that were first listed in 1972 in his monologue "Seven Words You Can Never Say on Television". The words are: shit, piss, fuck, cunt, cocksucker, motherfucker, and tits.
I can understand why shit would be considered a dirty word. Shit is dirty so the word for it should be considered so.
I don't think piss can be considered a dirty word because piss is actually sterile. While most people think it uncomfortable to be soaked in piss, some fetishes, specifically Omorashi, involve sexual pleasure surrounding seeing, smelling and pissing on or being pissed on and even watching the natural production of it. I'm not sure how pissing off somebody is socially acceptable while pissing on somebody is not.
Fuck cannot be considered dirty because, well, it's fun. Of course, some extravagant fucking practices do manage to get things sweaty, dirty, or wet. But the act of fucking is completely natural and fun.
Some cunts can be dirty just as some floor carpeting or dinner dishes can be dirty. So, I can understand why some people would consider cunts to be dirty, especially if theirs happens to be in that condition.
Cocksuckers may or may not be dirty just as poodles, cars or the above mentioned dinner dishes can be. It depends on whether a cocksucker has had a bath recently. What the cocksuckers do, likewise, can be dirty if the cock they are sucking is dirty or not.
Of all the women in the world who have had at least one child, anyone having sexual intercourse with her afterwards will qualify as a motherfucker. I am a motherfucker because my first and third wives were mothers when I fucked them afterwards.
Tits simply cannot be dirty. I don't find them particularly desirable, but I cannot see how they are dirty unless the woman possessing them has not bathed recently.
I like the word fuck because it has so many usages yet it's just a word. Fuck yes, fuck no, fuckin' A, fucked over, fucked up, fuck you up, I'm fucked, I fucked up, fuck you, fuck that, for fuck's sake, let's fuck, and, well, that's about all I can fuckin' think of.