The lock and key theory was dreamt up in Liverpool, England by an un-named genius. It states that a guy is like a key, and a girl is like a lock. A key that opens many locks is a master key, yet a lock that is opened by many keys is a shitty lock. This theory is rarely favoured by females... for some reason.
The lock and key theory is most suitably used to shut a girl up that complains about male sexual promiscuity being praised by peers, while that of a female is frowned upon. Works every time.