This society is obsessed with categorizing. We like to have everything and everybody in one place, neat and tidy. One thing.
People fall in love with other people, not their genetalia. We fall in love with people for their personalities and our experiences with them: how they treat us, their likes, their dislikes. If they treat you kindly and you know they love you and you truly like their personality, it doesn't matter whether or not they are male or female, because you have fallen in love with who they are as a person, not their genitalia.
Now I don't want you to seem like I'm disrespecting your preferences. Everyone has their preferences. But take this situation for example.
Say a girl has liked a couple people in the past who also happened to be boys. She then, whether she is aware of it or not, categorizes herself as straight and nothing else, because the people she liked, the personalites she liked, happened to be male. She then catergorizes herself away from ever having feelings for a girl, and if she ever does fall in love or have feelings for a person for their personality who also is female, she will push it away and ignore it because she has catergorized herself as straight, nothing else.
I am not saying that suddently its just like you can't be more attracted to those of a certain sex, its just that if you are a boy and you like a person who also happens to be a girl, that doesn't mean you are straight and thats that. Liking someone, loving someone, isn't straight or gay, it's just human.
One example of this is from BBC's Sherlock TV series. If you don't know, this is a modern version of the Sherlock Holmes tale, but it still has the same characters, etc. The two main characters are Sherlock Holmes and his, for lack of a better word, assistant, John Watson. In the BBC Sherlock, they are also roommates. John has previously liked people who also happned to be women and actually probably has more of a preference to women, he is more straight than he is gay, to, well, categorize it. But he falls in love with Sherlock, and not for his genitalia, because he is a person and he fell in love with another person, regardless of gender. And Sherlock has previously liked people who also happened to be men, and has more of a preference to men, and he has also fallen in love with John, but that doesn't suddently mean he can never have those human feelings of love for a person who is also a woman. When people hear that I (and many others, many, many others) know that John and Sherlock love one another some say: What! But John isn't gay! But that's not important, thats not the point. It is human nature to fall in love with other humans, regardless of sex, and John loves Sherlock.
According to Greek mythology, humans were originally created with four arms and four legs and a head with two faces. Fearing the power they would be capable of, Zeus seperated them into two beings, who would have to search the world looking for their other half. That other half may be a man. It may be a woman. Someone is... your other half because of who they are. Because you are a human and you need another human. Regardless of gender.
I will leave with this. You have your preferences. I have more experience having feelings for and prefer boys. But you shouldn't block out love just because of their gender or just because you think you have to be one thing.
Humans fall in love with other humans.