Responses
Because a bi person can "fit in". They can choose a "normal" life and be accepted by society whereas gay people will for the foreseeable future live in the shadows. This is in the context of Japanese culture and society. Filial piety, LGBTQ+ discrimination and all that play a big part into this trope(? of gay people being scared of Bi people leaving them to marry into a heterosexual passing marriage.
Amen.
Why are gays in bl always afraid of bisexuals?