D&D limits
NUDITY IN BOOKS, ETC > As far as rulebooks and supplements go, the amount of acceptable nudity etc. depends upon the book. In the core rules? I say keep it clean (no nudity and so on), as these are required books for all ages to play the game. If it is an adult-oriented supplement and the subject material calls for it, then go for it.
NUDITY/SEX IN THE GAME ITSELF > Sure, although no real graphic explanations unless absolutely necessary. Usually I like to keep it to "ok you guys hit it off, you head to your room...next morning you are smiling"

. However, one time my DM had to actually describe a sexual scene in a little more detail; my scout character stumbled upon two people in the woods at night...and being dark it wasn't immediately obvious what they were doing (although after his description it was

). So I guess I'm in favour of more detailed descriptions depending on the context.
ROMANCE/PREGNANCY> One of my campaigns featured a PC having a one night stand with a barmaid, but then falling in love with her, marrying her and then getting her pregnant. We basically kept it all in character as far as we were comfortable, which wasn't too far but far enough to get the point across and make it believable.
NECROPHILIA/BESTIALITY> Frankly, I never really considered using these as a plot device in any way, as I just couldn't see a reason for them. Both make me rather uncomfortable (and nautious), so I doubt I will except in the absolutely most vague implication (and then only if it seems really necessary for some reason).
INCEST> Well, again I hadn't really thought about it, although historically a lot of the nobility tended to engage in it to keep the bloodline "pure", so I guess I might allude to it in that context. That and Belial and Fierna seem to have that implied in the BoVD and I like to stick to D&D "canon" for fiends. But any frivolous or gratuitous use of incest would turn my stomach.
CHILD ABUSE> Again, this really upsets me so at most I would keep it to vagueness. Mostly character backgrounds and the like. I've played a few Call of Cthulhu adventures (
Fade to Grey comes to mind) that dealt with this in detail and it really upset me, so I'd definately avoid it in my campaign.
VIOLENCE> Really, D&D feature violence quite prominantly, and as such it's going to be in my games. However, I usually shy away from really detailed gore.
TORTURE/MUTILATION> Again, not something that I'm really comfortable with but it does come up. One of my PC's lost several fingers to a torturer and one of my own characters threatened it (he was bluffing though).