This collection is for het stories in which at least one character in the het pairing is bisexual/pansexual/omnisexual.
Characters don't have to be bisexual in canon as long as they're written as bisexual/pansexual/omnisexual in your story.
It's fine if stories have some slash or threesome content, but this is a het collection: the primary emphasis of the story should be on a relationship between a person who identifies as male and a person who identifies as female.
Stories about a character leaving a het relationship for a slash one that's the real focus of the story probably don't fit here. Use your own judgment.