Now then. This will eventually lead to some romance. But as I'm a sucker for the slowbuilding stuff, this might take a while. So, bear with me!
(Also, did I mention that bonus points are awarded to those who review and/or correct my quite awful english?)
SO WHAT
In which there is a woman in the living room and Sherlock doesn't really give a shit.
John had brought home a new lady friend. Normally, this would not be a problem, but this woman was not like the other ones that had come and gone. Statistically Johns dates were approximately seven years younger than him, blonde, thin and would generally be considered quite attractive. This one wasn't like that. Sherlock could smell trouble coming his way.
She was sitting in his, Sherlock's, chair by the fireplace, waiting for John to get ready for a night out. It annoyed him. Sherlock was in the kitchen, squinting at her in silence and making his endless deductions. First of all, she was older than John, but only by a few years. This was a new thing, as the man seemed to only care for younger women. Also, she could not be deemed a classical beauty, and frankly, not a beauty at all. Her face was asymmetrical, one ear slightly higher up than the other, and her eyes was just a bit too far apart. She was wearing a practical and unflattering blouse and a knee long skirt that indicated a profession in the educational system. Sherlock found it tasteless. But there was that thing that he suspected John thought was charming. She was cute.
Getting rid of John's lady friends was an art form Sherlock had perfected over the years. He had designed and tried out quite a few strategies that were infallible. Sometimes he could scare them of in a day, sometimes he took his time. He experimented on them. It was actually quite fascinating, and Sherlock carefully logged his results for further reference. What exactly made the women tick? What would scare them off? How much could they take before giving up? But mainly, he had realized, it all was a part of his longest ongoing experiment: John.
It wasn't anything personal in it, he just wanted his friend unattached and available. Girlfriends got in the way. Working cases was so much easier with a military doctor as back up. And really, John didn't seem to mind, not in the long run at least. There would usually be a few awkward days of not talking, mood swings, and no tea, but then they were inevitably back to normal again. Until he brought home the next woman, of course. The procedure would be repeated every fourth month on average, Sherlock calculated.
Eventually John came down the stairs and the woman anxiously turned her head towards the door before he came into the room. When he did enter, her face broke up in a warm lopsided smile, and Sherlock could once and for all confirm that yes, she was cute. John smiled back with his patented flirty smile, and Sherlock cringed inwardly. Not again.
"Mary, have you met Sherlock, my flatmate?" said John and gesticulated warily towards the kitchen. She shook her head, not tearing her eyes from him.
"No, we have not met officially," Sherlock filled in, and advanced into the living room with all the grace and dignity he could muster while wearing his dressing gown. He shook the woman's hand, and John developed a worried look on his face. The woman Mary did not notice, and smiled back at him too.
"It's nice to finally meet you! John has been telling me horror stories about you, but you don't seem that bad, really! I'm looking forward to getting to know you!" At this Sherlock raised his left eyebrow at John, who refused to look up from the floor, ears reddening. He coughed.
"Well, we're off! I got us tickets to the Opera. Mary and I have been talking about it for ages. Don't wait up, Sherlock!" They put on their coats and left. John shot one last glance towards his flatmate as he closed the door behind them.
Sherlock stood by the window and saw them walk towards the tube, hand in hand. He sighed, relieved that they had gone. There clearly was something special about the daft woman and Johns irrational affection for her. She seemed unquestioning and loving in ways he suspected John secretly admired and longed for. This one would be troublesome to get rid of.