Stories that depend on a twist in the tail have to contend with the possibility that some readers will guess the ending. One can deal with this by adding several Red Herrings, demoting the plot in favour of character or mood, or writing a thriller where there are several twists. There's another way though, that keeps both the naive and world-weary reader happy while retaining the "well-constructed" craftmanship. What you do is write a story with a twist as usual near the end. Experienced readers may well have anticipated this twist, but the writer needn't worry, because while the readers are gloating and off their guard you twist again. The first twist masks the second. It's a misdirection trick sometimes used by magicians - they let the audience think the trick is done one way, then they overtrump the trick.
It's hard to pull this off. Ideally you need to make the naive reader miss the clues to the first twist, and make the expert reader miss the clues to the second. However, readers vary hugely in their ability/desire to see clues, sometimes missing obvious points. Studies have been done to see how adaptable readers are and how their interpretations change from one paragraph to the next. At the start they often don't know the gender/age of the characters, the setting, or the genre. Some readers cling to their initial assumptions, others remain non-commital. If a first sentence read "What's your name?", Sam barked. you might assume that Sam is an angry man, though perhaps Samantha turns out to be a curious dog. See What Happens When Our Students Read, and What Can We Do About It? for a report on how a bunch of people read Graham Greene's "The Second Death".
In Agatha Christie's whodunits there's often a sequence of false endings, some rather unconvincing. When you Twist the Twist, the first twist must be good (satisfying enough to be a final twist) otherwise the 2nd twist won't work. Mamet's film "House of Games" has something like the same form.
The readers who keep an open mind and are happy to keep it open may not be suprized by these revelations, especially if they're experienced readers used to anticipating literary punchlines. But having 2 punchlines offers 2 chances to surprise the reader, and even experienced readers seeing the statement about the age may be lulled into believing that this is the final punchline. With their guard down they may be unprepared for the gender news.
I think in general that surprises work if they're seen to be
In this example I think the trick works because the author isn't assuming that the reader expects all relationships to be heterosexual, but is assuming that the readers expect an isolated fishing community to have repressive sexual mores.