Sherlock’s misconceptions about Harry Watson’s gender are important.
Pilots are where shows begin. It’s the setup. Additionally, the pilot of a show is written to gain an audience. That means that every small detail has been planned to achieve this goal. It’s all calculated and important to the eventual reaches of the show and the tone that the writers aim to set for it.
By making Harry Watson a lesbian, Moftiss is already squelching heteronormative ideas. Period. You think that because this person is married to a woman, they must be male? Wrong.
They would not have included this detail in ASiP were it not important; that is, if it did not say something that they felt needed to be stated.
And then there is always the other argument. “No, it’s probably because Harry is most often a male name. He would have thought of anyone named Harry as male.” But then, I ask, why name the sister Harry? Why would the writers, who are planning the beginnings of a Sherlock Holmes series that has never been done before and are rather excited there, give John’s sister that name? To make a point.
Because as attached as we all are to these characters, and I’m not discounting myself from that either, we must remember that at the end of the day, they are still characters. They are incredibly and heartbreakingly human, but they are creations nonetheless. Their thoughts and actions are planned; there are no mistakes. It didn’t need to be “natural” for Sherlock to perceive a person named Harry as male. But the writers decided that, for whatever reason
Sherlock is gay John is bi they’re in love, it was important that Sherlock be wrong in a heteronormative oversight.