A binary is a system that only has two parts, two possibilities. Whatever the topic, humans have a tendency to categorize people, ideas, and almost anything else as belonging to either group A or group B, in order to make the world easier to understand.
In regards to bisexuality, it is important to understand that the prefix “bi-” in bisexuality refers to having both same-sex and different-sex attractions. It is not in any way a commentary on the number of genders available in a society. Meanwhile, people’s tendency to think of sexuality itself in terms of binaries is a major source of biphobia.