I've found a solution for John's girlfriend problem. Since they all seem to be driven away by how much time John spends with Sherlock, the potential girlfriends need a change in perspective. John could advertise it in his blog.
Basically John is Sherlock's babysitter. So when potential girlfriends manage to view John as a dad, he should manage to keep a girlfriend. After all, you can't be angry at a dad for taking care of his kid!