Where do I say "unacceptable?"
NUDITY IN MY GAMES: Well, none of the players strip nude (Thank God) so nudity is restricted to such things as a player saying: "I'm going to make sure my claim-stone is in my necklace pouch after I strip down so I can go to the steam baths." or me saying: "The nude woman runs out of the bushes and begins babbling at you with an exhalted look on her face." I don't do descriptions of breast size and stuff like that. Although, once, I was asked if "the curtians matched the rug" when they were looking for an assassin with dark blue hair, and rumors had pointed them to the "Pervasive Joy Streambaths"
NUDITY IN MY BOOKS: I spent a lot of time in Europe, and nudity doesn't really bother me. To me, having nudity in books can either be tasteful and applicable (Some of the ancient religious paintings depicted women with naked breasts and/or buttocks) or immature and gratutitous (have a Hustler Centerfold painting inside of a creature book). I might put it in one of my books, if I think it is applicable, but for the most part, heroes are pretty much fully clothed (armor, bracers, rings, capes, girdles, boots, hats, etc etc etc)
SEX IN MY GAMES: Handled tastefully. It happens. PC's pick up on NPC's and vice versa. It's not a campaign center, but it's handled tastefully.
SEX IN MY BOOKS: No thanks. I'm buying/writing a gaming book, not a porn novel.
NECROPHILIA: Two actual categories. I do handle it, my villians are often dyed in the wool creatures of evil.
Sex With The Dead: Some of my villians do engage in necrophilia. Once with a dead player character to desecrate the body in a manner most foul. I don't describe it or anything, but some of my players investigate like they are part of a magical CSI. One PC has ranks in Profession: Mortician, and Profession: Autopsy, and Knowledge (Biology, Anatomy & Law) and that player will ask: "Was the corpse sexually attacked in life, or in death."
Sex With The Undead: One of the Legendary Characters (A 43rd Level Mage) was hit upon by an Elder Lich King (female NPC, but still the correct title was King) who was not only undead, but all the decayed flesh had been removed, and the bones plated in precious metals. We're talking a creature that had been alive for over 10,000 years, and ruled a vast empire.
Seriously, what was the PC Mage supposed to do. The player said: "Ick." but the PC said: "You offer intrigues me, shall we retire to more discreet locales?" and it was dropped from there. Some villians have undead lovers, some undead villians have living lovers.
But we handle it like adults, not with disgusting and/or graphic descriptions.
BESTIALITY: Never really come up, but there has been hints to it here and there. One PC was described as having an "Unhealthy Attraction to Sheep"
INTER-SPECIES SEX: As in Elf/Orc? Hey, we're talking intelligent creatures here. I know people in real life, that if given a chance, would have sex with a kobold just to see what it was like, why shouldn't some of the NPC's enjoy interspecies sex? One PC was attracted only to Orcs, Half-Orcs and Humans. A halfling.
PREGNANCY: Hmm. It happens. Since some PC's get married, it does occur. How it is handled depends upon the alignment and personality of the character. The CE female Blackguard simply had it aborted. The LG female wizard, who was married, refused to adventure, and the player got to play a temporary character while time passed in the campaign world.
But I'm not going to describe what a birth looks like. I've watched 3 kids being born. No need to traumatize my single players.
HOMOSEXUALITY: Hey, PC's and NPC's and on some occassions, players have been Bisexual or homosexual. Handle it like an adult, without being insulting, or zip the lip.
INCEST: Brought up now and again. The ruler of the Empire of Von-Lon killed his father so he could marry his mother. The Emperor's sister killed the mother, and took her place, leaving a married brother/sister set of rulers. Not uncommon in history, why should it be uncommon in the setting?
POLYGAMY: LOL In the Kargahst Empire, women own all of the property besides weapons and war material. They often have 3-6 husbands. Same with the Emirates of Kalavash, where women have harems of men. Of course, it is also the other way around in many other countries. In some places in the setting, there's what is called a "Bonded Grouping", which is 2-10 husbands and 2-10 wives ALL married to one another (this is a nation where bisexuality is normal). Polygamy is not that big of a deal, and since the end of the Second Lich King War, where there are fewer men, it is not uncommon for several women to share a man.
VIOLENCE: Ummm, it's a game that for many people, centers around hacking someone's liver out.

Violence is there, but we don't use it gratuitously.
TORTURE: Happens. Bad guys are evil for a reason. ANd some "good" churches engage in it against "heritics". It happened with the medieval churches in our world, it happens with some of the churches in my fantasy setting. I in no way encourage the players to do it, but they have.
Including our Modern d20 Game, where a combination truth serums, hallucinagenics, electricity and a knife was used to extract the location of a nerve agent dispenser in Seattle.
MUTILATION: Hmmm. Define mutalation. Is it: "Now that I've killed that @#%@#$%, I'm cutting off his head and sticking it on the end of my lance! @#*$&^@. I hated him!" or "The King orders the traitors head to be put on a pike outside the castle walls." or "The man is missing an ear and a hand, and sidles close to you, an unnatural gleam in his remaining eye." type?
CHILD MOLESTATION/ABUSE: Personally, I try to aviod this topic. A PC who engages this get the player looked at seriously. I don't care if he's won a golden plated apartment complex for his gaming skills, the PC's are supposed to be heroes, not child rapists.
NPC's, however, may engage in that activity. Not described, but the hint: "Markus Illavius is listed as having bought several dozen children in the last 2 years from the Red City slave auctions. These were prepubescent children, usually exotics from other lands." is usually enough to end up with that NPC being on the recieving end of some violence, brutality, and maybe even torture.
MENTAL ILLNESS: In a world with psions, mind affecting magics, and creatures that attack the mind, not to mention the Second Lich King War and the 9th Humaniod War, mental illness happens. Milkpond Weed helps keep paranioa (even magically or curse inflicted, including just physical damage) under control.
We've addressed it, and the players aren't horrified.
SACRIFICE: I have rules for Blood Sacrifices and other ritual practices, and it occurs in the campaign setting. The PC's don't engage in it, but they have stopped many a cult from sacrificing someone.
IN CLOSING
For the most part, I won't do anything in the game that would make my players uncomfortable, and even though I could write Vile stuff that would cause people walking by the bookstore to burst into flames, and people just logging onto the internet to have thier eyeballs melt, doesn't mean I am going to.
The question is not: Can I do it. Instead it is: Should I do it.
I can strip naked and dance on the Interstate with a black plastic bag over my head. Doesn't mean I'm going to.
I think that's the biggest line I have. Don't make the players or consumer uncomfortable.