@xris,
When a man wishes to marry a woman, he is required to give a dowry. This is usually a big financial commitment and a token of loyalty given by the man to honour and care for his wife. In the event of a divorce, the wife keeps the dowry.
For women who wish to abuse such a system, it is easy for her to marry multiple times, repeatedly get divorced and subsequently keep the dowry given by each man. Of course, this is unacceptable. As such, women are given less freedom to demand a divorce than a man. There is no financial interest for the man to divorce his wife, as he will lose the dowry and continue to have to pay to look after any children he has had with his wife. If he divorces and marries again, he has to pay another dowry to his new wife. With this system in place, it is more likely that the man will concede to a divorce once all efforts to save the marriage have been made.
Divorce is the most disliked of legal things in Islam. Islam teaches many things to make sure a husband and wife live happily together. But where a woman has a right to divorce, and is refused this by her husband, she can go to the courts and demand one. The courts can force the man to divorce if it is found that it is not reasonable for the marriage to continue.
There is equality in that the man must give the dowry, feed and care for his wife, but has the right to initiate the divorce. The woman does not have to give anything to marry, is not required to work but must go to the courts to demand a divorce. As with many things in Islam, the man and the woman have different rights but are still equal.
This will help you understand why it is prescribed in the Qur'an to resort to physically reprimanding a wife who is not fulfilling her Islamic obligations in a marriage. Note that this is a last resort. The 'beating' is not actually physical harm, as noted by the saying of Prophet Muhammad (peace and blessings be upon him).
This may seem oppressive to women but it should be understood in the context of a functioning Islamic society. Where there is a greater financial investment made by a man to marry, where there are fewer men than women, and where orphans and single women can become a burden on the public treasury, every effort must be made to keep men and women in a state of marriage. A righteous couple would never have to resort to such actions, as all the other teachings of Islam regarding marriage will have created a happy environment.
So, the follow-up question: is a woman allowed to physically reprimand her husband if he does not fulfil his Islamic obligations in the marriage? The woman has not been given this right by Islam. Trying to 'beat' her husband would result in him initiating a divorce, and so the action would be of no benefit.
This rule makes no sense in a liberal Western society where there is less emphasis on gender roles and marriage. The whole setup is different. To be fair to this issue, you should not isolate it as a snapshot and draw your conclusions from there. As Muslims, we take example of how Prophet Muhammad (peace and blessings be upon him) treated his wives, and he never beat them.
Regarding female slaves, their male masters are permitted to have sex with them provided the slave agrees. But seeing how slavery has been phased out, and this was no doubt the intention of God in the Qur'an due to the many verses calling for the freeing of slaves, this rule is not practised anymore. Slavery is no longer required.