My head canon has been that the equerry in asib is Mycroft’s boyfriend as they are dressed alike. Thinking about him instead as a possible date for John works too, as they are emphasizing his similarity to Mycroft. And Mycroft is clean-shaven (hence gay) in tab, therefore the equerry is gay.
This changes the palace scene from male gaze, isn’t lesbian sex exciting, put down your teacup John – to discretion is necessary, you understand why the princess may want to remain closeted, queer solidarity.