I think the fact that showing some of it is a felony might have something to do with that. Same reason the age of the characters was bumped up a half dozen years from what they were in the books. While marrying off a 13yo was common in medieval times, as you correctly point out, we take a much dimmer view of it now.A lot is mentioned, but few is actually shown. They even nicely kept their clothes on for the incest part and an HBO show without naked hookers is pretty hard to find.
I've so far seen 3 episodes of it, and yes there's quite a bit of nudity and sex. But it doesn't bother me much, because it always seems to serve the story. The bloodshed is also rather tame. Yes, there were a few beheadings, and one guy getting his stomach sliced. But it's not like they are constantly spraying the camera with showers of the red stuff. I like it so far, it feels mostly like a medieval drama. I have yet to see heavy fantasy elements, apart from the fictional setting and three dragon eggs.
Recently DL'ed the first four episodes. They're vying to be my evening entertainment, in direct competition with a dies natalis reception and my little pony.