I'm a white guy and I love bm/wf amateur porn. I have no cuckold fantasies or issues about penis size or anything like that...I just think that the best amateur porn out there tends to be the white-wife-getting-hammered-by-black-guy variety. Like others have commented, it seems that the participants get way more into it than you usually see in porn, and that makes it much more exciting.
I myself quite dig black girls (used to date one, and she was *amazing* in bed - pelvic muscles that could crush walnuts!
). I'd love to see more amateur wm/bf material, but even so, all of my favorite amateur porn vids are bm/wf, simply because they get into it so much.