either the one who says it's a choice IS Bi but had tons of shitty relationships with guys and no longer trusts them or she chooses to believe that her journey to discovering she was just a lesbian was a conscious choice rather than an involuntary realization due to trial and error
probably the former
As far as I and guliano are concerned MOST people are actually Bisexual but have been conditioned to think Heterosexuality is the norm by society, strictly gay and straight people do exist, but most humans are not gay or straight
our reason for believing this is that prior to the rise of Judeo-Christian religions, being Bisexual was considered the norm in most cultures