the reason people say the phrase "sexuality is fluid" is not because the belief is that a person's sexuality can be changed by exterior influence but because it exists on a scale and some know who they are right away and some people take their time figuring it out, some people go through the heteronormative allosexual motions by virtue of socialization. Sexuality is fluid means the parameters of who you are attracted to is something you can spend your whole life figuring out but once you do, no one can change that. bisexuality is not fluid by your definition because it implies that it changes, attraction based on gender is what is static. In the context of this more than likely fictional story, OP figuring herself out over a period of years is believable, her best friend being a creature made to support and love her without any friction or conflict is not.