The bisexual flag.
Bisexuality is when a person is only attracted to two sexes/genders (typically male and female). Of course, bisexual people may be attracted to two other sexes/genders; it's up to them to decide. However, the most common type of bisexual is the boy/girl kind. Other people who identify themselves as bisexual are attracted to more than two genders/sexes, but that's better defined by polysexuality or pansexuality. As with all sexualities concerning multiple genders/sexes, people may have a preference over one or the other, and that's to be expected.