The word which Jonathan Ross said to David Cameron was not especially rude, certainly not by the standards of anyone under 35. The activity represented by that word is not "lewd and obscene" by the standards of anyone at all. Most people would regard it at the same level as the other shocking and obscene word beginning with the letter W that Ross used in the same interview: "having a wee". A perfectly ordinary word to refer to a perfectly ordinary thing. Of course you'd be embarrassed to be caught doing it in public. (In that respect it's a lot like reading the Daily Mail.) But it doesn't make you short-sighted or drive you mad. (In that respect, it isn't.)
Satirists, like court jesters deflate pompous people. That's their job. One way of doing that is to imagine them performing private bodily functions. Didn't Winston Churchill overcome stage fright by imagining that his audience were all sitting on the lavatory? But mixed in with the anglo-saxon language, Mr. Ross was actually making two quite good political jokes. First, he insinuated that Mrs Thatcher was popular because Conservative men found her sexy. This isn't true, any more than it is true that John Major wore his underpants outside his trousers. But it's funny because we feel that Mrs Thatcher's huge popularity requires some kind of explanation; and because we can imagine that middle-class ex-public school men liked to be ordered around by a deep-voiced woman who thumped them with her handbag. You can't watch that footage of William Hague making his first political speech at the age of 14 without thinking that something weird is going on. But the joke is that Ross asks the question directly – were you ever sexually interested in Mrs Thatcher? Having embarrassed his victim by saying out loud what everyone has sometimes thought; Ross asked an unrelated question and received an evasive answer. So he repeated the previous question using much ruder language, adding "See, I'm just like Jeremy Paxman." Any fool could see that he was now making a joke against himself ("I'm the sort of person who would ask a rude question like that?") and against political interviewers" ("I'm going to repeat the same silly question over and over again.")
So how is it that when the Daily Mail affects to be very, very shocked by this lewd and obscene language both the Tory party and the BBC nod their heads sagely and pretend that it is a very serious matter. Should politicians go on programmes where they might be asked lewd and obscene questions? Should the BBC pay nineteen million pounds a year to someone who asks such lewd and obscene questions? Or are the BBC perfectly free to transmit lewd and obscene programmes -- even though of course I myself didn't watch it. Why did they not have the guts to say "Oh, shut up you stupid, humourless, holier-than-thou, frigid, late-Victorian, prudish, priggish, puritan."
The answer to the question "Why did a serious politician appear on this man's show" is presumably the same as the answer to the question "Why do the BBC pay this man £19,000,000?" -- because his viewing figures are nothing short of obscene.
Were we really so innocent that we missed the main point? The Mail, by pretending that it didn't know what masturbation was, managed to get us to file the programme under "The interview where Jonathan Ross made a rude suggestion to an M.P." This cleverly deflected attention from the fact that Ross also asked a perfectly serious question about Conservative drug policy. By responding weakly and evasively to Ross's perfectly valid points, David Cameron came across on prime time TV as a complete wanker.