Exactly how steamy is this story?
NC-17
There's a reason Game of Thrones is on HBO premium cable, and that is because you can't get away with the sex and steaminess of Martin's novels on basic cable. The FCC would shut them down, and there would be nothing any of the five kings could do about it.
A quick rundown of the graphic X-rated content in A Clash of Kings includes but is not limited to: sex, rape, prostitution, orgies, incest, and an age of consent that is basically non-existent. Oh, did we forget a sorceress birthing shadow assassins? Because that happens twice—or maybe she just had twins.
But the sex isn't just about having sex for sex's sake. At least, Martin doesn't think so. Speaking on the subject, he noted: "Well, sex is a huge part of life—it's a huge part of human nature, part of history and people's motivations. I think excluding sex is excluding a very important part of human nature" (source). In theory, then, if we understand what a character desires, and what they are willing to do to satiate that desire, we can better understand the character. This includes sexual desires.
For example, Tyrion pays Shae for her services, but he also is faithful to her and insists she only sleep with him, as well. This shows us that Tyrion's desires are not entirely sexual. Instead, he is motivated to have a relationship with a woman based on mutual love and faithfulness. Since we do not get to see the relationship from Shae's point of view, we can't say if this desire is reciprocated, leaving us, like Tyrion, to wonder what her motivations are.
Alternately, Ser Gregor Clegane, the Mountain, seems motivated by one desire only: to inflict as much pain on others as possible. He murders, pillages, and tortures his way through the War of the Five Kings, and as told by Chiswyck, he also viciously rapes a woman named Layna simply because her father annoyed him. For a sociopath such as Clegane, sex is another way to hurt others, both physically and emotionally, and his pleasure comes from others' pain.
We can determine a lot about characters by paying attention to the sexual dynamics they are involved in, be it by choice or by force. So while sex is in this book because it is part of the human experience, it's also here to help us fully understand who we're dealing with on the page.