Space flight and space exploration has always been a bit of science fiction. Some people believed in obviously crazy ideas and had a hard time defending them before they actually became reality. So, in a way, it is a rather soft border, if there is any, between reality and sci-fi in terms of space. So, going by your example, a Dyson sphere is not only sci-fi, but is has seen actual and proper scientific investigation.
Concluding, I would be damn careful with "demoting" any question to sci-fi just because their content may be beyond some peoples imagination. So they definitely must be allowed.
(And yes, there will be possible dumb questions about e.g. sci-fi television series, to put it mildly. But what the hack, they deserve a proper answer. Dig deep and you'll always find proper sources in this discipline. Sci-fi inspired scientists and engineers and vice-versa.)