He probably has a network of highly-intelligent independent women who enjoy not having the father over-interfering in their lives.

Possibly even a literal network -- like, if you do something brilliant in your master's thesis on engineering (or British equivalent), and you have a certain sort of dating profile, then as you're on your way out of the university, a woman you don't know hands you a card with an address on it ...

Meanwhile, John eventually makes a pass at Sherlock, and Sherlock, with genuine regret, says, "It would be lovely, John, really, but I can't justify wasting my sperm in such a way when it could be used to increase the intelligence of the next generation."