In addition to my previous comments, one might ask about the nature of celibacy. While it is not immoral ethically, it is touted in the Catholic Church as a superior vocation to those who can observe it as superior to marriage based on St. Paul. No doubt, marriage is considered a sacrament and the bearing of many children a source of blessing to fulfill the commandment to be fruitful and multiply. In Catholicism and Anglican theology, divorce is not allowed even though some marriages lack love and respect and even are abusive.

In Islam the primary purpose of marriage is love and mercy and having children is a blessed by-product of this union but divorced is allowed but is considered by Allah to be the most hateful in His sight of things permitted, so like marriage it should not be entered into lightly.

It is a pity that those who choose celibacy as do not always observe it properly to gain the favor of God and also do not enter into marriage to provide children who may be God fearing and contribute to the enrichment of the  society and the gene pool. The loss of such dedicated men and women is to be regretted.

To consider that those who separate or divorce except for adultery and remarry are committing adultery is an interpretation of what Jesus is supposed to have said that needs to be reviewed.

Protestantism recognized this only hundreds of years after Islam and also allowed married priests.

Polygamy, like monogamous marriages can have deficiencies but both should be allowed but when either do not fulfill their purpose and the bad seriously  outweighs the good, there can be dissolution of the contract.

Monogamy is the ideal, but many women remain unmarried and choose not have children. Some married women are unable to have children and rather than divorce may accept a plural marriage that allows her husband to have part of him to identify with rather than adopting.

We certainly cannot question the morality of the union of Abraham with Hagar as immoral when encouraged by Sarah for him to have an issue. He also fulfilled God's test by being ready to sacrifice Ishamael, his one and only son(at that time).

Certain commandments given to the Israelites were no longer accepted by the Christians even though they accept the Old Testament. In the Qur'an, most of these commandments were not considered binding on Muslims and were either modified, cancelled or replaced by others. 

The question of religious freedom in a secular society like the US as well as in Turkey where I am now living is a matter of heated discussion but polygamy may still be preferable to adultery and divorce.

The abuse of children, especially by a step-parent, is another down side of divorce.

Not to allow polygamy but to allow fornication, adultery, multiple marriages and divorce without any restrictions is to my mind immoral.