I suspect that on Mumsnet it's because everyone actually agrees that these are terrible crimes and so there isn't much to discuss.
However, in wider society, I think there is an odd lack of discussion about this. I think it's about changing mores, and the fact that we don't want to admit that we HAVE changed.
When I was growing up in a working class neighbourhood the 1990s, it was seen as 'normal' for working class girls of 13 and 14 to be having sex with older men. (It was very, very definitely a class thing). They were not seen as children in need of protection from predatory men - instead, men had a right to them. Most of the girls at my comprehensive school were regularly assaulted, on an almost daily basis, by teenage boys who felt that they had a right to grab us and put their hands up our shirts or down our skirts or on our legs. I'm sure I'm not the only person who remembers this! Teachers weren't much better - two of them at my old school have been jailed for sex offences against young girls.
Clearly, a similar culture permeated the entertainment industry in the 70s and 80s!
Thank goodness this has changed and we have begun to see these things - quite rightly - as terrible crimes. However, I think we don't want to face the change that has happened. I suspect that this is because we don't want to admit that society was more relaxed about the sexuality of children thirty or forty years ago, because we want to maintain a comfortable fiction that it has ALWAYS been the case that it's been as taboo as it is today, even though it clearly wasn't a while back, hence the public existence of things like PIE. I think we're also confused by the fact that we've been sold a narrative about sexual 'freedom' being a good thing, and Victorian 'repression' being a bad thing, and that these cases actually question that and point to the fact that certain kinds of 'freedom' can conceal a deeply patriarchal culture of oppression, which ignores the need for consent on the part of both women AND children for sex to be truly free. I think that this is therefore connected in a deep way to the discussions we have about Ched Evans and other issues - because the more we stand up against a whole culture of objectifying women, the more we will stop things like Rotherham from being tolerated.