This is my opinion. Asian men have a kind of virtual "Kick Me" sign on their backs, at least in popular American culture. Sexually they are seen (incorrectly, IMO) as geeky, soft, small and decidedly unsexy: this enduring stereotype plays against my personal experience, as does the Mandingo/Big Black Bull stereotype. But Asian men fight an uphill battle, both among the genpop and undoubtedly amongst themselves.
Everyone loses when negative stereotypes and prejudice pigeon-hole people into roles and attitudes that are distortions of reality.
Sorry, just don't see this much anymore. Jackie Chan, Jet Li, Chow Yun Fat, and Ken Watanabe are Asian actors who have made international stardom, and certainly do not have a kick me sign on their back.
I really can't remember the last time I saw the nerdy stereotype Asian you mention. As for Asian female in porn, I think they're stereotyped a bit less than they have been in the past. I see alot of BDSM with dominant Asian now and it's increasingly popular.
I think it has alot to do with the culture. They don't want to do anything that will bring shame upon their family or 'lose face.' I recently read on another site that Evelyn Lin, Asian-American pornstar, has been talked badly about in Chinese media and by Chinese netizens for "bringing shame upon herself and family."
I also agree with the statement that if the majority of viewers are white, then the top selling porn would be white performers. Just as in Japan the vast majority of porn is Asian on Asian. There are a few blakc guys in JAV, but white guys are very rarely in JAV.
I think as time goes on, we will see more Asian guys in USA porn, as it becomes more accepted in their cultures, and they become more assimilated into American culture. They may try to hold on to their old world ways, but I think assimilation is inevitable