I'm going to give a contrary view for you Pancho, just as an alternative viewpoint; it's obviously none of my business what you or your daughters actually do.
Thing is, however much you might want them to not do so, your daughters are going to continue to watch Game of Thrones somehow now they have started, if for no other reason than the story is compelling (whether watching on telly, dvd or torrent).
They are also going to want to watch it, especially if "Everyone else is", if only to not be seen as outliers in their peer group(s). In that respect GoT is, culturally at least, the TV equivalent of the trashy fiction that used to pervade my school years - seemed to be split on gender lines; boys reading incredibly violent and pornographic stuff like Sven Hassell* while girls I knew were passing around the Virginia Andrews books, rife with rape, incest and child murder - I think that GoT is significantly superior to either of those genres and at least has morality and justice at its core.
I would agree that GoT is more gratuitous in use of sex and nudity at times in a way that say, Rome (another very adult HBO production) wasn't - there was oodles of sex and nudity in that but it worked within the story (in my view). However, I do think that perhaps the most useful thing you could do would be to explain which/what issues you are concerned about with your daughters and talk through those matters. I appreciate I'm not a parent and YMMV, I just think that that approach is more powerful and respectful of them than an outright ban (which they will ignore and you can't enforce).
* I think it might have been Mr Larrington or Rogerzilla who said “Whoever said you can’t judge a book by its cover has obviously never read ‘The Bloody Road To Death’ by Sven Hassell”.
