Bisexuality in mangas

With the popularity of Yaoi, Yuri, BL, GL, Crossdressing, Gender Swap and Genderbender stories, I do wonder why Bi+ representation in these stories seems so hard to find.
I am aware that Bi erasure is a thing worldwide, and often their stories and experiences are simply pushed into straight or gay tags... but like... why? Historically, bisexuality seems to have been accepted to some degree, and there are a bunch of stories, myths and legends from various sources that does not put importance on the gender/body itself, but rather the person.

Am I ignorant of some secret lingo or slang for this topic? I search bisexuality on here and nothing came up...

Please, help me find the treasure trove (that i am certain exist somewhere) of people who are just as easily swooned by a beautiful woman as they are by a hot guy.