who's sleeping with who?

I have come to the conclusion that I should get a life. And I would if I had time. But for now it'll have to wait until I have had my say about last night's episode of Sherlock.

Yes, yes, it was brilliant and all that but for some time I was unable to concentrate because of something Sherlock said at approximately 4 minutes and 24 seconds into the episode. I know the time for sure as I just watched it again - and again - or the BBC iplayer. To quote, he said, '... or who's sleeping with who ...'

You see my difficulty?

Now normally I wouldn't worry about something like that - no, really, I wouldn't - but the episode had started with Sherlock correcting a prisoner's grammar over and over again. So I ask you: would a man particular about grammar say who's sleeping with who?

Even though my Good English Guide suggests that 'whom is being progressively dispossessed', I do not believe a man of Sherlock's grammatical fibre would so casually discard it.

There I've had my say. I'll go and start looking for that life now.