Maybe the sex wasn't that great.
The more I think about it, the more weird it seems that Joan and Mycroft had the sex. I’m with Sherlock in that I can’t put it into context. Joan seemed to be clear that Sherlock’s obsession with his brother’s vengeance plot regarding “bedding” her was bullshit, as was his tacit permission for the same. She rolls her eyes and ignores him. She also tells Mycroft it’s not going happen when she suspects that’s what is being set-up. But apparently it did. It makes me want to go back to the ep in London and see if it was being set up somehow through lighting or camera angles or whatever the heck (though the actor herself likely didn’t know it at the time).