Since the dawn of time, this character has been strictly heterosexual until one day he meets the man that will change everything. While he’s happily in love with his new partner, he makes a point of saying that this is the only man he’s ever liked and every single other person…
No. You can’t say this. It’s like no stop erasing biracial people because a person wants to identify as only black. Very poor analogy but it is what it is. Which is also what some characters are. Sometimes they are what they are. You can’t define them if you’re not the writer. Hell as a writer sometimes I can’t define them because the character is often revealed to me fully formed and they are who they are. Soooo no. If my character is bisexual or pan or het or whatever then they will be that. If they have only found attraction to one person from a particular gender then the reality is they are that and the flow of dialogue will ring true to that.