. . . is wrong in a "new and interesting" way.
That blog post (1) explains how the study went wrong and (2) asks whether it could have possibly been right.
As to the second point, the blogger (Ben Goldacre) explains:
It’s a difficult analysis to design, because in each age band, there is no information on gay people who are not yet out, but may come out later, and also it’s hard to compare each age band with the others.(The comments section on that post also has a lot of relevant insights.)
This reminds me of the oft-repeated factoid that "50% of marriages end in divorce." How could you ever determine whether this is true? You can observe divorces that have actually happened, but you can't possibly know whether existing marriages will end in divorce.
Even questions that seem to be about concrete, observable facts can't necessarily be answered by empirical research.