However an explanation is that a key that can open everyones locks is a great key a lock that can be opened by everyones key is not much of a lock
More boringly there may be evolutionary advantage for the females genetic perpetuation, for her to be selective and only mate with the genetically fitter males. for males they get best chance of perpetuating their genes by mating as often as possible - although prioritising the genetically better females (unless he is from Thailand and his formal partner cuts his plans short)
