If you haven't seen the movie or read the comic yet, there are spoilers ahead. I would recommend watching the movie first, and then reading the comic to see everything that the movie misses, and then finally reading and critiquing my post :D
As a hardcore (no pun intended) fan of the original graphic (no pun intended) novel who was a little disappointed by the movie but pleased overall, I feel compelled to respond. I understand that aesthetic opinions are subjective and that a work of art which does not present its rationale thoroughly enough is arguably not well made enough, but in this case the original work is just so dense and sophisticated in its presentation of its ideas that I doubted (rightly) that enough of it could be successfully translated to the screen. Is there gratuitous sex and violence in the film? There is a lot of brutal violence, and there is a fairly explicit sex scene, but I argue that it is not "gratuitous" because there is an extremely good reason for it being depicted in this manner. (People may disagree, but that's what debate is about.)"Watchmen" is simultaneously a sophisticated critique of an overmilitarized Cold War society and a hardheaded deconstruction of comic books and superheroes, all encapsulated within a bleak, cynical, sardonic view of the world. It strips the superhero naked (literally and figuratively) and tries to study the motivations behind an individual's decision to don a mask and prowl the streets of an alternative crime-ridden, poverty-wracked Manhattan. Read the graffiti on the streets? It says "Who will watch the watchmen?" Quis custodiet ipsos custodes? A quotation from Juvenal, with the idea taken from Plato. Who truly controls those who control? What do we do if those who are supposed to maintain order take liberties with it? What happens when the gods go berserk?Every single person in the book is seriously messed up, and in one way or the other takes to the life of a costumed vigilante as a way to escape from their personal demons. One person, the narrator Rorschach, is a sociopath with a deeply messed up childhood and an utterly Manichaean view of the world, who believes that his true face is his mask, and who is ready to inflict horrible violence in order to restore order. Another guy has trust issues with his father and lacks in self-confidence (in many ways) without his leather suit. Is there a sex scene in the movie? Yes, but it is a sex scene that is meant to parody the hypermasculine image of the superhero, and designed to illustrate the connections among sex, violence, and leather suits. These "heroes" are alienated from society through their constant exposure to the lowest, crudest elements of humanity, and through the horrifying orchestras of violence and brutality that they witness and compose. Which one of us would remain completely sane after witnessing first hand the animality that lurks under the veneer of civilization?There is a great deal of violence in the movie, but there is a purpose to it. It strips away the surgical cleanliness imposed by censorship on superhero comic books and movies—when the Hulk tears through an armored regiment or when Superman brings down a few buildings, do you really think nobody gets hurt? The violence is not meant to titillate but to shock; anybody who enjoyed the depiction of violence in the movie should be in a mental institution. The violence is as brutal as anything in Saving Private Ryan but more shocking, perhaps because of its immediacy; regardless, the goal is not to glorify blood but to depict the seamier side of superheroes, to show us that their violence actually has consequences that would be considered animalistic in real life, and also to show us the blowback that such violence has on the 'heroes' themselves.And that brings me to one other thing about the movie that has been noted by many, and criticized by some, movie critics: a few scenes of male nudity on the part of Dr Manhattan, the only real superhero in the film with powers that are essentially godlike. I find these criticisms juvenile and think they utterly miss the point. Dr Manhattan is, in some ways, a cynical version of Superman: a being who exists on a plane far beyond anything even remotely human. But unlike our kryptonite-allergic friend (again a foolish comic book invention that utterly turns me off from Superman, and which is also parodied in the film), Dr Manhattan acknowledges that from his exalted superhero position, human beings have no more meaning than termites. If so, why wear clothes? They are a foolish human convention after all. Greek sculptures celebrating the beauty and perfection of the human form aren't exactly clothed; it is modern Western society that seems to be afraid of human nakedness and sexuality, but really how would these matter to a being who can manipulate the structure of time itself? For what it's worth though, the movie and novel suggest (through the awesome scene of the shattering of the crystalline "Manhattan on Mars") that it's still worth it for us termites to continue living and feeling our emotions and struggling with the vicissitudes of our lives, and that the meaning of human life is not reducible to quantum mechanics.I've blabbed on for too long, but I wanted to make my point that the sex and violence in Watchmen serves an important artistic and, yes, moral purpose. Which inevitably leads me to my objections to censorship of art in all its forms: I strongly believe that it misses the point, it mars the artistic goals of the work, and, most reprehensibly, it treats us like children who don't know what's good for them. Of course, that's a much bigger question not directly connected to Watchmen, and I will refrain from saying anything more on the topic for fear of lengthening an already overlong, undercooked, incoherent diatribe.