LikeTearsInRain It's because it's a discriminating insult. Anybody can be a cunt, but you wouldn't call Owen an 'old bag' as only women over a certain age (generally post menopausal, so misogynistically considered 'past their best' ) can be called that. Therefore it's both sexist and ageist.
I've been on Twitter and Boy George is being absolutely trashed on there, mainly for causing people to feel sorry for MH, and lots of jokes about him wanting to chain him to a radiator.