I don't really think you can be born as a bi. It has to do with your personal feelings, and hormones could play a role as well.
So yes, you could say that it's a transition, and different factors play a role in it, such as the enviorment your in (your personal experiences, friends, etc..), your mindset and your feelings etc.
I truly believe that a lot of people are or can be bi, but it all depends on the above said factors.
For example, a girl that has a trauma for boys because she was raped when she was young, won't feel comfortable around any boy, but she will probably feel fine with girls. Only this one experience in her life could cause her to turn into a lesbian or bi (bi if she gets help and face her trauma, of course, something that will probably happen on a later stage).