Disclaimer: I own nothing.
Irene flirted with him. Shamelessly, he'd add, if it wasn't obvious already – people in her line of work weren't known for their modesty. Sherlock didn't react in any way. After all, she didn't know his secret (as it should be) so, no matter how insistent her offers were – and from someone who greeted him stark naked, the euphemism dinner for sex was laughable – the sleuth didn't react to them. She wasn't serious. She couldn't be.
If she'd seen him naked...John actually liking his wings was without a doubt a fluke – a very lucky fluke, indeed. With what she was used to in her work, Irene maybe wouldn't have called him a monster outright. After all, she must have had contact with people she didn't find particularly arousing, without letting them know. But she would be disgusted nonetheless.
And anyway, even if she claimed that she * wasn't * going to blackmail anyone, adding his own secret to her stash of blackmail material would have been so utterly insensate that Sherlock wouldn't have done that even stoned out of his mind.
Which he has been. By her. He didn't appreciate that one whit. The last thing he needed was having enough of a taste to go back to wanting drugs. He already wanted so many things he couldn't have. Thank God that he's really been fine as she claimed once his transport got rid of it. He'd have killed Irene if she got him addicted again. (John would have left him, disappointed in him. That's what he's done to his own sister, after all.)
Not to mention Irene scared him. He wasn't used to being unable to read people, and he didn't like it one bit. How was he supposed to behave without having an inkling about the other person? How did normal people manage to blunder through their life that way?
God knows Sherlock has been called reckless a lot during his life, but ordinary people are much more reckless he's ever been. They have relationships with people they can't see through... And then they go to him or another private detective to know if they've been lied to. Which they usually have been.
For a moment, upon seeing Irene for the first time, Sherlock had been absolutely terrified of having lost his gift – his deductive powers. His life would have crumbled if he had. Then John appeared, his many times over saviour, Sherlock deduced him and the panic receded. It wasn't him. It was the Woman (not that it was much better, having a blind spot like that). She was a puzzle he couldn't solve. A problem which took a liking to teasing him.
And John thought he liked her. He didn't trust her – he could have never trusted her – but this didn't seem to matter in John's book.
The only thing that seemed to matter to him was whether Sherlock wanted to make babies with her. (Which he wouldn't because a. she wasn't the motherly type and would probably rather get an abortion and go back to the life she enjoyed, and b. women really weren't his area, they were John's, but he didn't seem to see the difference between them.)
Or whether Sherlock was falling in love with her. Again, could love really exist without trust? Sherlock didn't think so. If he'd have to fall in love – which he wouldn't, not ever, he knew better than fall prey of chemical defects – it would be with someone with whom he felt comfortable sharing his wings.
And that sorely limited his options, did it not? There was only one person Sherlock could have imagined falling for, if he'd ever fallen prey to such an ordinary people's ailment, in truth. And that person was very vocal – repeatedly – about not being interested in him. But since Sherlock would never be stupid enough to misinterpret a chemical imbalance as anything related to sentiment, things were fine. They were more than fine. Just perfect.
Now, if John – for all his lack of interest in Sherlock – would have stopped behaving in ways he'd call jealous in anyone else (but that couldn't be, because John very clearly didn't want him; "Who would?" inner Mycroft gleefully chimed in) the detective wouldn't be so confused all the time.
John was a puzzle. A fascinating, never ending walking contradiction. And if not jealous, then what? Concerned? God no, that made him sound like Mycroft. Wishing he'd been the one to catch Irene's interest? Maybe that. Yeah, definitely that. It would explain why John was so irritated by Irene flirting with him.
Maybe he was still hoping to sway her affections towards himself, and that was why he was wondering how deeply involved Sherlock was. He wasn't the type to purposefully steal a girl and wound an acquaintance's feelings in the process.
Perhaps Sherlock should pretend to be a bit more affected than he was to play upon John's loyalty and keep him from pursuing the Woman. He didn't want John to date Irene. She wasn't safe (which made her just about perfect for the doctor – a bit too much perfect). No. Irene and John would never be a couple. Not if Sherlock had any say in it (and with John's love life, he usually ultimately did.)