A person who labels themselves under a certain sexual orientation, but is flexible beyond those restraints if they choose or if it is demanded of them.
My homie straight but in prison he was a flexosexual during is 10 year sentence.
Sarah only dates guys, but became flexosexual when Taylor Swift wanted to be her girlfriend she went along with it.
A sexuality in which one's sexual attraction changes over time, sometimes on a day to day basis, sometimes over the course of a month, depends on the person for the exact time. In labeling, one might just say they are flexisexual, or put another label for their main or most common attraction, or even put a label for the range their attraction goes through
"I thought she was hetero" "she was yesterday, now she is a lesbian, bet you anything she'll be all over guys again tomorrow, she is flexisexual, her attraction changes over time"