Some of these were confused people. You can actually repress sexuality and cause attraction to a different gender through hypnosis. Mass media constantly bombard you with subliminal messages, and modern social engineering is anything but healthy. All these environmental factors can lead to confusion and delusions about your sexuality.
Homosexual/heterosexual people who repress their sexuality and think they are the opposite. Bisexual people who repress part of their sexuality and think they are either pure homosexual or pure heterosexual. People with severe sacral chakra hang-ups can also be deluded into thinking they are asexual, which is not a real thing.
This can extend into past lives as well, not just your current life. Only through meditation and gaining awareness, as well as freeing the soul workings, you can reach a level where you are 100% sure your sexuality is free and authentic, rather than a product of brainwashing.