Replying to a very old post, but I will reply since I found the question interesting.
Firstly, although it is the case with most content on here, not all cuckolding is interracial, and not all interracial sex is cuckolding. In fact in both cases I suspect it is not. A white woman may cheat on her white husband with another white man, with or without his knowledge or agreement, a black woman may cheat on her black husband with another black man, with or without his knowledge or agreement. Same goes for cheating husbands...
Another angle I view IR porn (and I like white male with black female too) under is as a fetish for race mixing in general... Most cultures, in my opinion, have always (and still do) pushed against mixing with other ones, even of the same race, although to various degrees.
