Although not sex-related, my parents would say that everything from wanting to dye my hair to multiple colors (such as black and orange) to wanting tattoos or piercings to the types of shows I wanted to watch was "just a phase"....yet those "phases" haven't changed for the majority of my life!
I think often, parents who don't want their children to think or feel a certain way about an issue chalk it up to a "phase", as a personal hope that eventually it'll all just go away. Which I think is silly.
Hah, being bisexual, just a phase
You woke up one day and said, I think today I will be bisexual
lol that is the type of mentality they would've had to go through, were my theory not the case...but who knows, after all, so many people also believe it is a "choice", meh, could've been either I suppose :/