I suppose it depends on how many dozen crass comments they had to read, in part, and perhaps it was left to someone new to the marketing department to do the boring bit, who may have felt embarrassed to ask about the 2 girls 1 cup reference, just because it seems 'cool' not to need to ask...
They have had the marketing people criticised as being incompetent for not asking about it, and how "hip" they probably felt they were being to use social networking site but that this has probably been their worst nightmare.
I'd still point the finger very much in the direction of the agency - the Coca-Cola people must have indicated their target audience, and I bet the agency was left to do "the rest", so they came up with the messages... If Coca-Cola came up with the messages, then I'd point the finger at them, after all, the client is always right (and the most an agency might do is point out a possible error, and keep their name well away from the campaign if they felt it was "dodgy").
Every week I hear or see something "out of the ordinary" (ie unknown to me) which may be slang of an offensive kind. Had never heard of a "Chelsea Smile" until this thread. Had never heard of "crack one off" (until I was up late and it was said on Big Brother).
There must be 1001 expressions of varying levels of crudity which we, as individuals, may never have heard.
Regarding 2 girls 1 cup, I had not seen a thread about it on MN, nor did I even know the name, but someone on geeky_stuff enquired about stopping her son from viewing porn via his mobile or the net {sorry, some months ago} and I happened to have seen something on C4 or Five about teenagers, where that clip was described.
The presenter had been told what was in the clip by two teenage (14-15) lads, and I could hardly believe what I was hearing, yet the lads seemed quite unconcerned (presumably having seen other filth, too).
As with the "reactions" clips (about seeing 2G1C) on YouTube, there was (within the TV show) a part when the parents saw it, so we as TV audience could see their reactions.
Don't remember anyone being actually sick, but most of the Mums covered their eyes, and looked ill. The Dads also showed signs of revulsion, but in all cases, it was a massive shock to them that their sons had seen such revolting muck.
So what I'm getting at, is the likelihood (as per a poster's comments much earlier) that perhaps someone young in the agency included it, thinking it "acceptable" to use that line, and the agency had a duty of care to come up with (suitable) goods!