I dunno where they’d meet, but Sherlock would have others SDs apart from John, and although the others might not know that they aren’t really Sherlock’s only SD, bc you know, they all think they’re special, after all that’s Sherlock’s job, but John would always suspect, and be a bit bothered by it, but he wouldn’t say anything, of course. And Sherlock would eventually get bored of each one of them, specially after he met John, bc well, not all of them are like John and John really gives him everything he needs (money wise, and gifts, and attention) and he’s hella attractive, which always helps. And he’d drop the other ones, one by one, without even noticing, and John would be the only one to remain and Sherlock wouldn’t ever get bored by him, and John would be at his happiest when he realized Sherlock was only seeing him