The thing is, I'm also an adult who doesn't (currently) play with children, and I have no problem with R-rated art in and of itself. Nor indeed do I have any inherent issue with more explicit material, in and of itself. Still, I don't want present in the D&D core. (I would, however, be fine with it being present in supplements that are clearly marked as being for "Mature Audiences".)
This actually ties in quite nicely with my opinions about sexism in the artwork, and since I can make the argument more clearly that way, that's what I'm going to go with.
Here's the thing: my wife is not a gamer. However, she is a big fan of "Lord of the Rings", is fairly well-read in fantasy literature in general, was recently caught reading the latest Pathfinder book, and is indeed a bit of a geek herself (but don't tell her I said that). In many ways, she is exactly the sort of person that WotC should be aiming to recruit as a player - someone interested in the subject matter, and who might, with the right push, become a gamer.
Now, I don't think that the artwork in the game should be tailored specifically to market the game towards her. To be honest, I'm not actually sure what that would really mean. But what I am quite certain of is that WotC shouldn't fill the books with artwork that will drive her away. So, if the books are full of wall-to-wall cleavage, if every female character is barely dressed and posed for maximum effect, and if the female heroes are constantly being depicted as being in peril (with the big, tough men around to save them, of course), she'll push them aside, roll her eyes, and never take another look at the game.
The extension of that towards the "age rating" of the artwork should be quite obvious - I don't necessarily want the game aimed at 10-year-olds. In fact, that would almost certainly be counter-productive.
But I have a nephew who's nearly six, and who one day may become a gamer. Another parent in my group has children well on the way to becoming gamers. That's the audience of the future. WotC don't need to aim the artwork for them... they just need to not include artwork that will prevent me from giving my nephew a PHB when the time comes.