Well Japan is still a largely homophobic country so the reaction and worry about bi partners is fairly understandable. They're trying to say that the pressure to fit in with the rest of society, bi partners have the choice to build a family with a woman rather than be prejudiced against being with a man. It's a valid concern for them.
unless you're bisexual, you're not the one to decide if it's biphobic rhetoric or not. on its own, thinking that bi people are more likely to cheat/to "play with gay people's feelings"/etc is biphobia. it stems from stereotypes and oppression towards fellow queer ppl for " not being queer enough". even if the character/person isn't actively biphobic all the time, this entire line of thinking IS.
this entire justification of biphobic line of thinking among gay people is the reason why many bisexual people only ever date other bisexuals because being in relationship with gay/lesbian people has a huge risk of them being discriminated against. despite what others think, being bisexual has no real privilege over being gay.
the "but he's bisexual" bit was so unnecessary mannnnnn can gay guys in manga ever be normal about bisexual partners...