Gender Equity in the Qur'an:
Men and women are equal in the Qur'an. This is stated on countless occasions. "Indeed, the Qur'an is full of verses affirming the equality of women and men. I defy patriarchy's theologians to reconcile these verses with any bias against women or even relegation of women to a sheltered -- that is, inferior - status," or denial of the right of Muslim women to lead the Ummah (Dr. Rifat Hassan):
Never will I suffer to be lost
The work of any of you,
Be he male or female:
Ye are members, one of another
[Surah 3:Al-'Imran:195]
If any do deeds
of righteousness, --
be they male or female --
And have faith,
They will enter Heaven,
And not the least injustice
Will be done to them.
[Surah 4:An-Nisa':124]
The Believers, men
And women, are protectors,
One of another: they enjoin
What is just, and forbid
What is evil: they observe
Regular prayers, practice
Regular charity, and obey
God and His Apostle.
On them will God pour
His mercy: for God
Is Exalted in power, Wise.
God hath promised to Believers,
Men and women, Gardens
Under which rivers flow,
To swell therein,
And beautiful mansions
In Gardens of everlasting bliss.
But the greatest bliss
Is the Good Pleasure of God:
That is the supreme felicity.
[Surah 9:At-Tawbah:71-72]
Whoever works righteousness,
Man or woman, and has Faith,
Verily, to him will We give
A new Life, a life
That is good and pure, and We
Will bestow on such their reward
According to the best
Of their actions.
[Surah 16:An-Nahl:97]
For Muslim men and women, --
For believing men and women,
For devout men and women,
For true men and women,
For men and women who are
Patient and constant, for men
And women who humble themselves,
For men and women who give
In Charity, for men and women
Who fast (and deny themselves),
For men and women who
Engage much in God's praise, --
For them has God prepared
Forgiveness and great reward.
[Surah 23:Al-Mu'minum:35]
In Surah (9 - 71-72) above, men and women are protectors one of another. This is an important distinction for those who argue that women are weak (intellectually, emotionally and physically) and must be protected from harm by their male counterparts. You can't take verses of the Qur'an on their own, you have to view the revelation in its entirety and if you do so, you will notice that men and women are equal.
You asked specifically about marriages in Islam and in particular polygamy. The following verses pertain to the question of polygamy in the Qu'ran:
"And if you fear that you will not act justly towards the orphans, marry such women as seem good to you, two, three, four; but if you fear that you will not be equitable, then only one, or what your right hands own; so it is likelier that you will not be partial (or become destitute)." (4:3)
"You will not be able to be equitable between your wives, even so you be eager. Yet, do not be altogether partial so that you leave her (i.e., the wife discriminated against) as it were suspended. If you set things right, and are God-minded (or godfearing), God is All-forgiving, All-compassionate." (4:129)
Some argue that the second verse is an abrogation of the first. The line "you will not be able to be equitable between your wives." But most scholars believe that the status of polygamy in Islam is no more and no less than that of a permissible act. Not an act encouraged, but rather a rare exception. And, like any other act lawful in principle, it becomes forbidden if it involves unlawful things or leads to unlawful consequences such as injustice. Again its a matter of taking the Qur'an in its entirety and not just plucking out verses which suit your fancy. The Prophet curbed abuses by limiting polygamous relationships. In doing so he signified the harm in such relationships.
It could be effectively argued from reading those verses that Islam does not permit polygamous relationship unless "equity" exists among the consenting participants in the marriage. The definition of this "equity" is the issue contention. Some would argue it is an impossiblilty to achieve such a state of equity, while others (mainly conservative's who cling to the interpretations of early male Islamic jurists) would argue otherwise. Many modern scholars have argued against polygamy quite effectively, using the Qur'anic verses.
Another argument made by modern Islamic intellectuals is that these verses (particularly the ones revealed in Medina) should be understood in terms of the historical conditions in which they were revealed. They argue that these verses should not be used any longer in the development of social and economic institutions - although they should not be discarded either. This is a radical view. The majority feel that the verses should be re-interpreted. Ijtehad is our right, we do not have to rely on the interpretations of past jurists forever - afterall they were only men and were prone to error like the rest of us.
In terms of divorce, what in particular do you find unjust in Islamic divorce?
Achtung ;)