Being Bisexual in fantasy only is completely valid! I am actually mostly that myself. I have very little romantic attractions to real people. I mainly fantasize about being in a relationship with some unnamed man or woman. Usually, there is a face, but it is generic and vague. I fantasize about being in a relationship, but I don't usually anyone specific in mind! I'm pretty sure this is very common. And if it isn't, who cares? Faceless biromantic/sexual fantasies still count as Bisexuality according to the definition give by Bisexual.org!
A bi person has the capacity for romantic and/or sexual attraction to more than one gender. For most people, that simply means that you can be attracted to both men and women. If you honestly feel you meet that criteria, then you are bisexual.
I think it doesn't matter if it is a choice. There is a discourse that sexual orientation is not a choice and that's probably true to a certain extent. But, would it matter if it was? Who cares if it is (epi)genetic or learned or due to hormones or a choice? Scientists do because that question is important to understanding the sexual nature of humans, but it shouldn't affect your own identity as a Bisexual/Queer person.
I think it is helpful to see myself as a person who happens to be Bisexual. I recognize that is a position of privilege and not everyone can feel that way. I don't think it matters. Just enjoy yourself responsibly and who cares what others think? The main reason I publicly talk about being Bisexual is so others can realize they're not alone. Sexual orientation is just a minor trait like eye or hair colour and has no direct effect on your personality or who you fundamentally are as a person.
I hope this helps.