As many whites and couples have admitted in the past, it comes down to two very simple reasons: taboo and contrast.
Taboo in the sense the American blacks are viewed negatively by many whites who constantly ridicule and despise them and just cannot view them as just regular americans. They don’t behave this way to other races or even other nationalities. Just very deliberately and specifically to American blacks.
Also, the color “contrast” they keep referring to is just a metaphor for the real contrast of seeing their all-american middle-class American wife having “raw”, “animalistic”, “pri.m.itive” (these are words whites have used, I’m just quoting) sex with well-endowed sexually-aggressive strong “blacks”. This “optics” or the shock and taboo of this is what keeps whites wanting to see their partners fucked by “blacks” or “the blacks”.