It is important for the Prime Minister to say that sexist jokes are wrong because it shows strong leadership and it sets the trend for a non-sexist society. Prime Ministers always need to be very careful about what they say, this is not the 1800s (or even past then) where sexism was accepted and no women were allowed into the House of Commons. We live in a modern and free country where feminism is coming into action. The Italian Prime Minister, Silvio Berlusconi, is known for having really bad attitudes towards women. We don't want that.
I don't understand why the MPs reacted so much when David Cameron said that Nadine Dorries was 'extremely frustrated', she was talking about abortion, a very sensitive subject and she has the right to get angry about it. Lots of male MPs get frustrated in the House of Commons and they are not jeered.