I recognise that it might be counter-factual justification. If I had explicitly wondered if "married/unmarried" were or were not exhaustive possibilities, I would have realised that the intent of the question was to treat them as exhaustive possibilities. The actual reasoning as I remember was "Only one of these people is known to be married, they are looking at someone of undetermined marital status". The step from "undetermined marital status" to "either married or unmarried" was not made, and, if you had asked me at the time, I might well have answered "could be divorced or something? .... wait wait of course the intent is to consider married/unmarried as exhaustive possibilities".

I am pretty sure that if the question had been

Three coins are lying on top of each other. The bottom coin lies heads-up, the top coin lies tails-up. Does a heads-up coin lie underneath a tails-up coin?

I would have answered correctly, probably because it pattern-matches in some way to "maths problem", where such reasoning is to be expected (not to say that such reasoning isn't universally applicable).