Also imagine what ran through Hitler's mind when he decided to formulate his opinions on Jews. It probably started with some limited interaction with or exposure to them, and it probably turned out negative (in your case regarding Asian women, it turned out positive). He then extrapolated that into a generalization... and then some unimportant things happened next. A negative fetish, if you will.
A wrong idea is bad, regardless of if it is a positive idea or negative one. It makes you disregard other Asian women, or other people, who actually hold those qualities, and it makes you think less of them when they fail to satisfy those qualities. I hope that helps you understand why such generalizations, even if positive, are racially insensitive.