1. I think women have an basic instinct to want to be bred and produce babies so it is the ultimate act of total surrender, enhanced by the taboo of interracial coupling. Even though the sole purpose for the existence of semen is to be deposited into a woman's womb and make a baby most responsible women choose to block it most of the time because real life makes it inconvenient to get pregnant. Amie has also let it be known that the is very submissive to black men and is willing to let them do anything to her so accepting their semen is a natural thing for her to do. Of course fear of disease would also dictate whether or not a woman will receive the gift of a man's semen.
2. Insofar as the black male is concerned, no self respecting alpha male would want to deposit their cum anywhere but inside a woman's pussy. The alpha black male has the same instinct to reproduce their offspring and it demonstrates their dominance over the female and other males when their sperm beats other male's sperm to the egg. Of course there are other ways black males demonstrate their dominance over the female lover and her husband such as performing anal sex on her.
3. From my perspective as a white husband of a desirable white wife it is a turn on on multiple levels. First of all, as Amie's husband I think of myself as the alpha male within our relationship and allowing her body to be taken without limits by black lover or lovers is an act of surrender on my part. The ultimate surrender is when black men cream deep inside her pussy, deposited their essence where mine is supposed to be. This ignites something inside me that fills me with desire to also cum inside her, to compete with their male potency. That is why I always make love to her soon after they cum inside her. I had given up my alpha male status for a short time and need to reclaim it as mine.
Initially, when we first let black men have sex with my wife I had assumed condoms would be used and was shocked when Amie willingly accepted their bare cocks. I have since grown to be turned on by the sight of bare black skin against white with result being my wife being filled by taboo black sperm.