Bisexual: noun. a person who is sexually attracted to both men and women.
Heterosexual people generally take this definition to mean "a person who is attracted to women fifty percent of the time and men the other fifty percent." It is the truth for some, sure, but it is in no way the truth for every single person who identifies as bisexual.
The main way this concept is visualized is the Kinsey Scale of Sexual Behavior. This scale defines human sexuality in a set of numbers from 1 to 6, from heterosexual only to homosexual only. The Kinsey Scale originates from both the Male and Female volumes of The Kinsey Report. These findings were made as part of an effort to describe people's sexuality and sexual experiences at any given time. This scale, although most know it as being from 1-6, also includes an "X" rating, for those who experience little to no sexual attraction. People who identify as heterosexual only, and even those who identify as wholly homosexual, assume that anyone who is attracted to more than one gender simply falls in the middle, at a three. There are so many more places to identify on this scale, however. One can be 60/20 in favor of a gender that isn't their own, or the same proportions leaning more towards homosexuality. The erasure of these numbers and identities is a huge problem in both the straight and gay communities.
The image above is a visual of the Kinsey Scale, and shows the broad spectrum of bisexual relationships. Why, then, do people erase the idea of bisexual being more than a 50/50 balance? The answer, in my opinion, is ignorance. People who are fully straight or fully gay don't understand the idea of being anything but one or the other, or right in the middle. In fact, bisexual people are often considered selfish, greedy, or slutty. These stereotypes do more harm than good when it comes to helping people understand the bisexual community, one that already lacks distinction among the rest of the LGBT+ community.
Being bisexual isn't being greedy. Being a bisexual person with an affinity towards homosexual relationships doesn't make you gay. Being a bisexual person that is generally in heterosexual relationships doesn't make you straight. Bisexuality isn't a phase. Being bisexual is more than just a trendy term used to make you a "special snowflake." Being bisexual is a valid and true identity, no matter where on the scale you identify.
Straight people do not get to define where queer people sit on the spectrum, end of discussion. Not-straight people are the only people who get to define their sexuality and gender. Boxing bisexual people into this box of 50/50 is both unfair, and incredibly harmful erasure that needs to come to an end, and quickly.