Sex
How much sex, and how blatant the sex, depends on the writer and the audience, rather than the genre.
Now, personally, too much sex in a novel bothers me becuase it can too easily interfere with the story. Except when it's really well done and a necessary part of the story. Real people generally take a crap every day, too, but I'd rather that be off stage, as well, unless the writer can convince me the story must have it.
Again personally, I've tried to read more than a few modern books that were labelled "erotica," and I've yet to find one that had any distinction at all from porn that I could see. On the other hand, I find some of the more classic erotica extremely well written.
But now and then I'll pick up a novel with pretty graphic sex, and it all works together, it's all part of a seamless whole, and the read is great.
I don't think it's ever a good idea to include sex because you think it will make the novel sell better. Odds are you'll write a novel that won't sell at all. Whatever teh genre, the right way to write a novel is to tell the best story you can tell, and fill it with the best characters you can create, and let the story take its natural course. If this means sex, it should always be because you want it there, and because the story demands it, and never, ever because you have the thought "You know, if I put a steamy sex scene in right about here, more people will buy the novel."
