watched A Scandal in Belgravia for the fourth time.. wait maybe fifth time, but just noticed something

It was the scene where John meets Irene in the strange abandoned wear house. They were talking and John says "I am not gay." and Irene says "But I am" or something like that.

(I can't find the script and my vlc player isn't working so I can't play it again) haha

But, what I am here to ask is.. why the heck did she say this? I mean, she is "SHERLOCKED" So, how can she be gay? I am reading the book but I haven't gotten to the Scandal in Bohemia yet. This is just bothering me now. HELP! :)