I think its an issue that cannot be ruled upon apart from as an appeal to common sense and goodwill. This is a game with players from across the globe which uses a common language to play, English : problem there is that there is no single 'English' with one unified set of meanings for each word : quite the opposite : the strength and power of English is that any single word can have multiple meanings and uses and each can have a different 'weight' where ever it is used. Its what gives English such poetic might, it feeds comedy and drama, writing and conversation : the allusion, the pun, the double entendre, the synonym, the rythym, the twisting of meaning, and as Shakespeare so wonderfully displays, the new-minting of words and re-purposing of old ones to new effect.
The word 'twat' is a great example : its original (anglo-saxon) meaning is simple : a cut or clearing in a forest. It dont take much imagination to see how such a word could become a crudely poetic allusion to certain body parts. It seems in some regions that is the primary meaning of the word and thus it causes offence: in mine, the words meaning has primarily been to signify foolishness or stupidity, as an emphatic play on the word 'twit'. Thus, for me, the word does not offend at all as it does not carry any salacious meaning and so I shake my head and wonder at all the fuss about it.
A similar word would be 'cleft', with similar ancient origins : to most, it climbers talk, or maybe refers to a damaged palate : to me, it means female parts : in my youth, thats a word we used, among others, for that subject : we also used 'kilt', 'hedge', 'box', 'front garden' and many others : all 'innocent' words but also, in that, context very rude and thus offensive to some.
And that is the only rule we could work with and make sense of: what is the context of the word, what is the intention of its use : is it meant to shock, to insult, to draw attention to itself?
But lets not chuck the baby out with the bath-water : the joy of language, of conversation, of 'banter', of comedy and laughter is the skirting of the edges of meaning and decency : the art of making the 'rudeness' be in our heads, in our interpretation of meaning and intent. remove that and life quickly becomes very very dull.
Finally : I adore the name 'Biggus Dickus' : yes, its crude, school-boy humour : but when I see it, I also see Michael Palins injured and confused innocence as he struggles to understand why all these people think his good friends name is funny. For that one moment of sheer comedic genius alone, the world can handle a little bit of being offended.