First off I have a high suspicion this study won't be reproducible. Secondly the TFA has this very interesting paragraph:
The Stanford University researchers found that gay men and women typically had "gender-atypical" features and expressions. While a person's "grooming style" also factored in to the computer algorithm, essentially suggesting gay women appeared more masculine and vice versa.
This begs the questions, how much did "grooming style" factor into the apps gaydar, and where did the list of gay and straight "grooming styles" come from?
On top of all this, TFA doesn't give any details on sample size and I find it difficult to believe they managed to get a large number of pictures of random people... along with the sexual preference of the person.
Thus spake the master programmer: "After three days without programming, life becomes meaningless." -- Geoffrey James, "The Tao of Programming"