lock and key

someone asked why is it okay for boys to have many girlfriends but not vice versa. a girl having many boyfriends is a blasphemy. and then someone used this analogy of lock and key. no its not like the enzyme and substrate lock and key hypothesis. a boy is the key and a girl is the lock. what happen when a key can open lots of locks? we call it master key but what happen when a lock can be opened by many keys? it is not a good lock. so you get what i mean? if im not mistaken my friend told me about this from amovie somewhere... cant remember...regardless of that it is still wrong for both sides actually  haha