Being in a relationship is a choice though. I don't have to be with my husband but I want to and not being in a relationship of your choice is ridiculous. When people say this to another person it's just cruel and stupid really.
However, sexuality isn't a choice. And I agree with the fluid thing or scale, I personally don't find females attractive enough to be intimate with, a mans body turns me on and that general attraction to their features. Emotionally also I just like boys better, my best friend is male gay, I have female friends too but yeah it's all platonic.
You're just born this way though, I didn't choose to only be attracted to boys, same way someone can't choose who they're attracted to and you can be on each end of the scale and somewhere in between. Bi sexual just means your attracted to both, it's not being confused your just wired to be attracted to either and it depends who you fall in love with in your lifetime. I have a couple good friends who are bi and have had long term relationships with both boys and girls. They're not confused, they just always liked both.
How else do you explain it? It just is.