Spoiler:
I think the nudity is a really important aspect of the film - a rare example of it being necessary - as it contributed to the sense of otherness of all these characters compared to norms of behaviour. Michael and Deirdre had completely different views of beauty, desire and sexuality compared to mainstream society and part of that was a voyeurist/exhibitionist trait. For Michael I also think there was an interesting comparison with the way he dressed when he was with Mary (I mean seriously, that tweed sports coat could not be more buttoned-up, suburban, normal). Philip and Abbey have a number of similar traits - the exhibitionism, lack of prudishness - and in many ways seem more conventional, yet their relationship is incredibly destructive. And he's the one who ends up with PIG written on his chest. Physical openness with each other did not give them any kind of emotional intimacy.
Apart from the fact that it contributed to the story, I also appreciated that it was pretty equal-opportunity nudity - because that's rare, even in films where it's warranted it's usually only the women who end up naked in front of the camera.
Apart from the fact that it contributed to the story, I also appreciated that it was pretty equal-opportunity nudity - because that's rare, even in films where it's warranted it's usually only the women who end up naked in front of the camera.