I read the article Korean slurs Japanese and Chinese culture and morality, which claims Chinese and Japanese are more sexually loose than Koreans. I don't see why that Korean guy is acting so high and mighty like Koreans are more chaste than any other people. It's not like Koreans don't have sex. There is ancient and medieval erotica from Korea, just as there is erotica from other countries. Human sexuality is part of every culture - or we won't be around, right?
But come to think of it, such attitudes are not new at all. Many prejudices didn't just arise yesterday. The following quote is from Lives of Eminent Korean Monks by 13th century Korean Buddhist cleric Kakhun:
[The Korean king] initiated the hwarang (a group of 'handsome youths' from the nobility, who were trained to serve the court as performers as well as warriors) ... The [Korean king's] aim was to make the people progress towards the Great Way. Emperor Ai (a former emperor of ancient China who kept many beautiful young men at court) ...loved only lust... This indeed cannot be compared with our story [of the Korean hwarang]
So anyway, even medieval Koreans believed that the Chinese were lustful and low-minded in their male-male relationships at court, while the Koreans were virtuous and high-minded. Yeah, right. Historical records refer to pederastic relations involving several kings of the Koryo dynasty (918-1392) [See p18 of Male Colors by Gary Leupp]. Let he who has no sin cast the first stone.
And as for the pre-modern Japanese, the word they used for Chinese/Korean person was synonymous with the word for stupid person. And the Chinese in old times, had a slang word for Japanese, meaning "short person".