It's interesting to see that people are equating the fact that we accept violence in a fantasy setting, as a reason we should accept rape in a fantasy setting.
We all play wargames where we roll dice to see if we maim, murder and mutilate our opponents - so would we all be ok if GW brought in rules so we could rape our opponents army as well?
I'm sure many forum members play computer games where you can shoot, stab, decapitate and slaughter other characters - but I doubt there's many who play games where you rape the other characters.
Fantasy and sci-fi are fundamentally meant as escapism, where we disconnect real life from the actions and motivations of the characters we play or read about. Rape is much more than an action, it's psychological and deeply rooted in emotion and empowerment, and hence it's difficult to disconnect our real life feelings from a character in a game or story.
I'm not saying rape has no place in fantasy or sci-fi, it absolutely does, but it needs to be handled sensitively and effectively, and it should be necessary to the narrative to warrant it's inclusion.
I've only read a few black library novels, and i'm not a huge lore buff, but my view of 40k is it's a pretty 12A-15 kind of place. With this in mind i find it difficult to see this diorama as anything more than a bunch of models stood around, and a very ham fisted(though fantastically presented) attempt at conveying a deplorable act.