Not necessarily. We have to remember that bisexuality is a sexual orientation on its own that deserves to be recognized. Bisexual people face a lot of prejudice, from society but also from the LGBTQ+ community. One of these prejudices is the preconceived idea that bisexuality is a phase and that these people can’t decide what their sexual orientation really is.
It is completely false. Bisexuality, just like homosexuality and heterosexuality, places itself within a multitude of sexualities where people are more or less attracted to two or more genders. Bisexual people might have preferences for a gender, this doesn’t mean that they have transitioned toward a different sexual orientation. “Just a reminder that heterosexuality can also be a step of questioning our sexual orientation, but we still don’t doubt its validity!” (unofficial translation)
On the other hand, bisexuality can be transitional for some people that question their sexual orientation, even if it isn’t everyone who, during their period of questioning, identifies as bisexual that will end up discovering they are for a fact bisexual.
Ministère de la Santé et des Services sociaux en collaboration avec le CLSC des Faubourgs de Montréal, Bien vivre son orientation sexuelle – Les femmes et l’homosexualité, 1999, page 7
Dubuc, Dominique (2017). Les mots de la diversité liés au sexe, au genre et à l’orientation sexuelle. https://fneeq.qc.ca/wp-content/uploads/Glossaire-2017-08-14-corr.pdf