It is true that in all Moslem countries today the practise of polygamy is well established. However the plurality of wives does not conform to the teachings of Muhammad. The Qur'an (Koran), on the one hand, appears to permit a man to have more than one wife, on the other hand, positively and unequivocally, states that this is conditional upon absolute justice among the wives. Since absolute justice is incomprehensible to man, let alone unenforceable, therefore it stands to reason, that Muhammad was saying that polygamy was not to be practiced. The Qur'an (Koran), therefore mandates monogamy, not polygamy, as has been understood and practiced to the present day.
It is evident from the above example that it is unwise to judge a religion, any religion, by the statements and practises of its adherents. A good maxim to follow is 'the independent investigation of truth'.