six is not completely unnecessary. Gratuitous sixual content is unnecessary.
However, complex sixual themes represented in a mature light (note: actual maturity, not just six for the hell of it or six outside the realm of art and taste) can work wonders in making a believable world. Dialogue and conversation and NPC reactions as displayed via quests that have sixual connotations or hints (such as the naked Nords in Morrowind, or the cheating spouses in Bloodmoon), objects like Boethia's Pillow Book, factions like how Dibellas' cults and worshippers SHOULD have been in Oblivion, etc, etc.
The same thing can be said for gore, drug use, and all the other things that the ESRB flags as apporopriate for an M rated game. They are not completely unnecessary, but there is gratuitous and stupid implementation, and then there is sensible, credence- and believability-building, contributing-to-the-overall-player-experience implementation. Rich, complex, emotionally gripping skooma dens and skooma addicts, instead of just a merchant who sells "DRUGS!" or some tacky "lol, these guys are all stoners!" implemetation. Well-placed dungeon gore/horrors that actually enhances the mood and builds the suspense and player sense of of heightened awareness and creepiness, instead of a "SPAM DEAD BODIES EVERYWHERE!" or "CUT SOME GUY AND WATCH HIS ARMS FALL OFF!" or "BLOOD SPURTS EVERYWHERE!" implemetation. Use in moderation. Oblivion had a crapload of visually dead and dark things, visual gore. Heck, look at all the world objects for the Deadlands, the necromancer lairs, all of that. And yet they spammed it so much and screwed up the overall mood so much that it still felt nonthreatening and "blah" despite all the gory and dark content.
And, as others have said, make the game as it is and then worry about rating, don't design the game around the rating.
I am, however suprising it may seem, largely in agreement with Thatoneguy here, and believe it should rate M for all the same reasons. I will go the step farther and say that there is no logical moral or propriety argument that can be well made for not allowing something as ubiquitous as exposed bums. . . and, for my part, I feel that some of the aspects present in the games should have more follow through. These things add to immersion, verisimilitude and plausible depth. Magameto went on at length about how such things as six should be absent in the game because a game could never adequately relate the complexity of a real relationship. . . in saying this he managed to conviently overlook the perpetual and ubiquitous nature of "casual" six (which of course, can also bind a type of relationship), yet he also conviently bypassed the fact that NONE of the relationships in the games are capable of rendering all of the complexity they would in real life. If actual interactions were as shallow as many of those in the game, you would be convinced that everyone around you was severely mentally retarded. A great deal of the "relationship" factor is dependent upon the imagination of the players. I have an in game "friendship" with my mage apprentice. Not because he communicates well. By normal "real world" standards. . . he is clearly mentally challanged, as his dialogue is WOEFULLY limited. He also does idiot things like run to assist me in battle with foes far beyond his caliber. . . which forces me to defend his idiot self, along with mounting my own defense. Yet the fact that I do this at all is evidence of our having a friendly relationship. . .taking my apprentice with me into Oblivion gates, or hidden shrines as part of the educational process. . . all of this is primarily, my imagination tacking addenda onto the games existing features. And until in game AI becomes FAR more advanced, all in game relationships, friendly, antipathetical, sixual, professional or other will be imperfect, less complex than real world counterparts, and requisite of some player expansion. That doesn't mean the games should just stop trying for verisimilitude altogether, and take out the food, the beds, the houses, the stables, the inns, shops, and everything else in the game that does not directly involve monsters, lightning bolts, and Daedric claymores. All that to say, it would behoove the game and its players for "mature" content, sixual and other, to be included in the games, so long as it is not done in a way so blatantly tacky as to be no more than a flashy gimick for attention.