Lubrican has raised an issue of racial -- I don't know how to categorize it. "Point of view" perhaps? Anyway, read his blog post to see it. It's worth reading. It's here: http://storiesonline.net/blogentry/34601
Here are my thoughts:
I agree with you that most of the adult stories seem to have white protagonists. Undoubtedly most of them seem to be written for white audiences by white authors. But "seem" is perhaps the crucial word.
There are a lot of stories that don't mention the color of the participants, let alone their race. We'll rule out the ones that describe the female as having a pale complexion or auburn hair with lots of freckles. But there are plenty that just describe her in non-racially identifiable ways: big boobs, cute butt, long sexy legs. Men too. I've heard it said that even some black men DON'T have huge penises.
We just assume that the mechanic named Fred who is balling his wife named Melissa, are both white. Does he have to be named Jerome and her name has to be Shaniqua for them to be black? Does the man have to be called Eagle Spirit for him to be Native American, or a woman Kim Woo to be Asian? Would a reader assume that a scene between Barack and Michelle was interracial?
Perhaps we're looking at the stories from the point of view of a white reader, who would automatically envision the protagonists to be people like him (or her) and the antagonists be someone "other."