She just wants to have the experience or the thrill of having sex with a Caucasian man, and lock it away in her memory. For the most part these women are educated, work respectable jobs and fit into Japan's homogeneous society.
Every man needs to realize that his needs are different from a woman's needs. As a man, you should take the time and make the effort to learn what makes your woman happy. Just because you got a woman to be your girlfriend, or even marry you, does not mean can stop making the effort to meet her needs. In fact, if you want your woman to stay with you, meeting her needs becomes more important than ever, or else she will start looking elsewhere for a man who can do the job.