At the end of volume 3, there are "interviews" with Aki, Asami and Fei Long. Asami evades almost every question and instead muses on his recent encounter with Akihito in "Body Chase." One of the few questions he directly answers is what kind of women he likes, to which he responds "I admit I am drawn to strong women on occasion." As far as I know, that is where the idea that he is bisexual comes from.
Sensei said it in an interview. I think it was Yaoicon or something.
Was there a q and a about Asami being bisexual?