I think that in both reality and fantasy when doing this kind of thing, its a huge huge turn on for both white wife+hubby or GF+BF when the black guy doesn't use a condom and cums inside her pussy - just for that less than 1% chance of her birth control failing.
Risk = reward.
The greater the risk the more of a turn on anything is. Obviously no one wants to catch an STD but for a long-term white girlfriend/wife to get knocked up while she's sexually active with both her white BF/hubby and black lover is the ultimate tease of a turn on for all involved - without a paternity test during pregnancy, waiting to see if the baby comes out white or black its the biggest excitement possible especially for the black guy for that ultimate "alpha male" feeling over her white BF/husband if the baby comes out black.
My wife has no risk to be pregnant because she is 46 ( and my mom is in the same situation...). However she loves to be filled by a big load of black sperm. She tells me that it is because she feels her vagina overstretched but it is difficult for her to explain precisely why. Unfortunately , I am not present when she meets his black lover in the afternoon . She tells me in the evening when we are quiet in our room.