Porn does seem to play a bigger role in shaping sex-culture and pop-culture than many of us would like to admitt...it is a major factor in shaping stereotypes. As you said, in [porn videos], these asian guys are disproportionately femme. If I may also add, not only are they mostly twinks, they're also all hairless! And now, these are the "official" stereotype of asian guys (however unconscious) in the western gay community. Whoever doesn't fit in the stereotypes is considered "an exception". (even though there are more asian guys who have many traits outside the stereotype. I guess in the webster dictionary, we should re-define the figure of exception to around 55%. lol) 
And it doesn't end there. Now, with these new stereotypes, other porn industries go out to search for the very asians who fit the 'asian stereotype' and reject those who do not. With this representation of asian guys in these newer videos, the original stereotype is only re-enforced. See the endless cycle?
For many gays of whom I've encountered, they'd make fun of the fact that I'm asian and that I'm hairy. Some even show a sarcastic smile and a subtle face of disgust ('What?? A HAIRY asian??'). That goes to say, if the hairless asian twink is not bad enough, now we have something even worse for these guys: a hairy asian semi-stud!!!!