
It is a thing that exists in the real world so the question you should be asking is can we stop people from being biphobic? So that the theme is not relatable just like with the homophobic theme but the answer is probably no because there are alot of assholes in all types of groups including bi or straits who pretend to be bi just to mess with people

Calling it biphobic is being blind to the societal context of this manga. It's kinda when people come out to their parents a bi even though they are very much gay/lesbians. Because society still expects for the w|m marriages and family. Specially so in countries and cultures that are not welcome/accomodating of queer people such as Japan.
Being bi for a gay/lesbian person would imply for them that they are most likely to conform to societal pressure and marry into a heterosexual passing marriage. For the gay/lesbian person that means a definite ending to their relationship with a bi partner, something that they may not be looking for, particularly if they are looking for a life partner. It's not biphobic not wanting to date a bi person when chances are they would chose to conform to society. It's protecting their heart and not going into a relationship with due date. Yes, no relationship is guaranteed, but the way they see it, bi people have a chance of being acknowledge that they don't have and chances are they'll chose that and that's why there's a reticence. It wasn't hate nor being phobic. It was a way of protecting themselves against heartbreak. It's not denying their being bi, it's not being able to cope with the what if. In this cases is more like a thing of preferences. Knowing that they won't be able to deal with such an scenario they prefer not dating bi people. And that's actually more healthy than entering a relationship were you would be second guessing yourself at every turn if you don't have the mental strength to fight your insecurities
can we stop the biphobic theme in mangas??