Because there are plenty of gay and lesbians out there, walking around right now, who are currently either in monogamous straight relationships or never have had any same sex contact so far. Also, vice versa with people who HAVE had some incidental same sex contact, but self-identify as straight.
What about people in that category - Their relationships and actions are not signs of their actual orientation, right?
Something as complex as human sexuality is really difficult for learned professionals to comprehend, imagine how lay people get confused and make completely idiotic assertions all the damn time.
Most people can describe themselves… But it gets damn near impossible when it comes to classifying other people.