It is possible for someone to have more than one sexual identity, or they can identify with something that’s not popularly recognized. You can’t assume that someone is or will always be straight, even if at the time they are with someone who is of the other gender than themselves. The person them-self might not even be completely sure what they like. For instance in the story “Frog Loves Christy” there is two sisters and the one sister, Frog, wants to become a man. After she does so Frog admits her love towards Christy who is already married to a man. They both struggle with this till Christy finally admits her love for Frog too.
Then there are others who aren’t sure so they experiment with each other, or by wearing clothes or makeup of the opposite sex.