I don't think anyone is saying Cersei isn't responsible, but you make it seem like she is responsible for Robert's upbringing of Joffrey, which is completely false.
Rudeness, excellent. I wasn't attempting to make excuses for him by saying those things, I was merely mentioning them because they are relevant. I wouldn't call his approach to parenting lazy; in other things, yes I would. Yes we have been reading the same books, but they are open to interpretation, and while my interpretation of a character might be different to yours, it doesn't make either of us wrong.
And on the topic of Roberts' bastards, they all seemed to turn out fine. Obviously that's down to their mothers, but they showed no signs of anything near what Joffrey was like. Which is why I blame Cersei more for the incest, and so the bad genes, and her parenting. Maybe I'm wrong in that, but her having children with her brother not only slightly disgusts me, but it also makes it clear that having Cersei as a mother is damaging. Coupled with Robert, it was a disaster.
@ David Selig: I agree with you, I don't know how many times I've said that both of them were to blame. I can see why Cersei was angry with Robert over the cat incident, but Cersei did nothing about it. This is what I've been trying to say, and have said before - both Cersei and Robert failed in their parenting because they were complete opposites of each other. Cersei saw nothing wrong with Joffrey, never saw fit to chastise or discipline him; Robert went over the top when he did discipline him, but neglected to take proper care of him as a father should. With two parents like that, I'm not surprised Joffrey turned out the way he did. But his cruelty with the cat shows that he was innately wrong. The mixture of the incestuous genes, Cersei allowing him to do whatever he wanted and Robert taking no notice of him, along with having no constant, good father figure in his life, led to him becoming pretty much a monster.