Write more than one bi character in the same story. It’s dangerous to let a single character represent All Bisexuals Everywhere™. (Same with any other character who belongs to a specific minority group.)
Does one character fall into the ‘bisexuals sleep around’ stereotype, but it feels true to who they are, so you want to keep it that way? Add another bisexual character who doesn’t sleep around.
Does one female bisexual character end up with a man, frustrating your wlw audience? Add another prominent bi woman who ends up in a sapphic relationship.
Have two bisexuals. Have three. Have twenty! Give them different gender identities, racial identities, body types, backgrounds. Real life bisexuals are all over the map, so why should you have only one?
