I know practically nothing about either genre, but have to wonder if yaoi might be more popular in Japan partly because the language doesn't use sex-indicative pronouns, so the reader isn't constantly reminded. And I seem to remember that there are at least some sexually-ambiguous first names, so it would be possible to get well into a story, much as some pioneering s-f writers just casually revealed that the protagonist was Black (or Asian) more than three-quarters of the way though the story.