Unfortunately, there isn't a ton of research on the topic that I can find. I know that some scientists agree with the fluid sexuality idea and others don't. There is a sexuality scale called I think the Kinsey Scale and a grid that incorporates more than just gay and straight which places sexuality on a spectrum and I think that is the direction most research is headed in.

You also have to consider sociological aspects to human sexuality. Maybe as a kid, I wanted to be with a boy because I saw adults in heterosexual relationships. I don't mean at all that seeing people in relationships makes you develop that sexuality, but that children emulate adults. I have a friend who has two moms. As a kid, her Barbie dolls had either two moms or two dads. She turned out to be straight. However, I know some people who say that they always knew they were "different" and always had feelings for the same gender. I don't know.

Also, if I (maybe you too) were to see a girl and experience a level of attraction, it's possible that we would not allow ourselves to experience that. It would be too confusing and we'd automatically shut it out.

Sexuality is interesting. I wish I knew more.