Many of the trolls complaining about a female doctor are semi-professional complainants who can be traced through a myriad of anti-BBC tweets, posts and rants. They are the same people who complain UKIP aren't given a fair hearing (for our overseas friends  - the racist group has been pretty much promoted by the BBC through their failure to scrutinise the 'party' to the extent others are held to account); these are the people who complain if they have heard something they might not lim has been shown on the BBC; the people who rant and froth when an alternative comedian pokes fun at the the ruling Conservative party, Queen, flag, Christianity (even though they without a Christian bone in their bodies), complain when a favourite presenter is scaled for punch out co-workers; complain if the ways of the country are presented on TV.
Some fans may feel uncomfortable at having a female Doctor even if they are happy to see one when they need antibiotics.