Ahh, unfortunately, my Japanese skill is not good enough to say this...and it's kind of controversial...a lot of Caucasian people might get offended...
Relationships and sex are one of the playing fields that race is played out upon. Caucasian, or white colonialism in Asia and elsewhere in the world puts them on top, in control of the region, for example, like in China before WWII, Shanghai was divided up into districts for the French, British, Japanese and Americans, who had economic power there because of China's colonialism. However, colonialism reflects itself in the minds of the people too. Caucasian men find foreign women exotic, and exciting, and in some case, see them as subjects (like in a colony), as submissive and obedient. Caucasian women on the other hand, often see foreign men as less than Caucasian men. Like in a colony, they see foreign men as submissive and weak, and not attractive. So Caucasian women don't find foreign men attractive. On the other side of the issue, Caucasian men are often powerful, have lots of money and so on, so ambitious foreign women often are attracted to Caucasian men. In nations with lots of Caucasian people, the television and media make Caucasian men the default manly standard, so especially Asian girls here see Caucasian men as the default for manliness, rather than Asian men. When you turn on the TV, you see lots of Caucasian males doing the heroic roles. However, the number of Asian men are few. Many Asians tend to play villain roles, or weak roles.