There are canonically-straight-as-far-as-we-know characters I can't see as gay or lesbian because we've seen their relationships and/or been inside their heads and I just can't see it. That doesn't mean I can't see them as bi.

As for gay or lesbian characters not being that, there aren't that many in my fandoms that I can think of as examples -- but putting them into het relationships or even saying they're bi in a story could involve a load of privilege and baggage I wouldn't deal with.

But in theory I can see just about anyone fictional as bi.

Real people is another thing entirely, whatever somebody says they are in real life I back the hell away from that because it's them. I can't even imagine engaging someone I know about their identity, wow, no.