ok ok so imagine genes like trading cards so two people have different decks of cards they make baby they play rock paper scissors for gender now listen for this to happen the man likes women the woman likes men now youll end up with one, both, or neither of those cards so if you are per say, male you might get dads sexuality, which is towards females or moms sexuality, which is towards males maybe both maybe neither you cannot choose for yourself, its just a big game of rock paper scissors loser gets rid of a card

its the same science behind eye colors you might be born blind (which often has no eye color, hence menaing neither) you might be born with moms eyes or dads eyes or Heterochromia both. now it isnt the exact same, like for eyes, its more common to have just one eye color than anything else. For sexuality everything is evened out.