Sexuality is a spectrum. This means it varies between one side:heterosexual and the other: homosexual. You can't be entirely on one side because there are people that find out they like the same sex later in life.

Attraction could also be towards their personality and not limited to appearance.

Consider it like a spectrum of skin color. Some people are paler than others while others are born more tan. Some are in-between.

I believe bisexuality is exactly in the middle, but obviously leads to one side just a little more.