All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If someone is asking to learn something about his religion, he has to be polite in his words. It is strange to consider yourself as a judge on Islam that it is right in such and such matters and wrong in such and such matters. Who are you to take this position of the judge?
Islam is the religion of Allah, the Lord of the Worlds, and it is His straight and sound legislation, none has the right to criticize Him; Allah Says (what means): {And Allah decides; there is no adjuster of His decision. And He is swift in account.} [Quran 13:41]
Therefore, what you said is very serious and you must repent from it or else it is a statement that necessitates disbelief in Allah Almighty, especially that you glorify the west and honor some of the rulings of the Jahiliyyah (Pre-Islamic Era of Ignorance) upon which its system is based. A person who believes in Allah and the Last Day does not do this.
You should know that Allah, Glorified and Exalted be He, knows what is better for His slaves in their religion and their worldly matters, because He is their Creator, so He is the One Who legislates for them. Allah Says (what means): {But no, by your Lord, they will not [truly] believe until they make you, [O Muhammad], judge concerning that over which they dispute among themselves and then find within themselves no discomfort from what you have judged and submit in [full, willing] submission.} [Quran 4:65]
If the Sharee'ah permits something, it is either because of its pure benefit or that its benefit is dominant over its disadvantage. The marriage with relatives is something that is proven with evidence that it is permissible.
Allah Says (what means): {O Prophet, indeed We have made lawful to you your wives to whom you have given their due compensation and those your right hand possesses from what Allah has returned to you [of captives] and the daughters of your paternal uncles and the daughters of your paternal aunts and the daughters of your maternal uncles and the daughters of your maternal aunts who emigrated with you…} [Quran 33:50]
The Prophet married Zaynab bint Jahsh, who was the daughter of his paternal aunt, and he married his daughter Fatimah off to his cousin 'Ali ibn Abu Talib .
The ancestors and Muslims from the past until now continued to marry their relatives, and many researchers stated that the issue of genetic diseases cannot be made a rule and that such diseases may occur and may not occur.
The disadvantages that you have referred to, which may result from marrying relatives may not occur. Even if they happened, then this does not mean that it is forbidden due to that, as there are advantages in this marriage, which we may know and may not know.
Marrying more than one wife is also permissible in Islam, provided its condition is met, which is being just between the wives.
Allah Says (what means): {…then marry those that please you of [other] women, two or three or four. But if you fear that you will not be just, then [marry only] one.} [Quran 33:50]
The matter is as we mentioned in regard to marrying relatives; what includes great benefits cannot be overruled due to some assumed disadvantages.
One of its apparent benefits is that the number of women exceeds the number of men at any time and place, especially at the time of war.
Limiting the marriage to only one woman leads to a large number of women staying without marriage; this causes them embarrassment, hardship and break up, and it may lead them to selling their honor and the spread of Zina (fornication or adultery), and loss of family lineage. There are other benefits in marrying more than one wife; if you look for them you will find them.
With regard to your saying that Islam allows child marriage, then this is a broad term, because marriage includes the contract and consummating the marriage with the wife. The contract with a young girl is permissible as evidenced in the Quran and the Sunnah.
Allah Says (what means): {And those who no longer expect menstruation among your women - if you doubt, then their period is three months, and [also for] those who have not menstruated. And for those who are pregnant, their term is until they give birth. And whoever fears Allah - He will make for him of his matter ease.} [Quran 65:4]
The verse indicated that the waiting period for a young girl is three months, and the Islamic reason for the waiting period is marriage.
It is confirmed that the Prophet married 'Aa'ishah when she was six years old and he consummated the marriage with her when she was nine years old. [Al-Bukhaari and Muslim]
However, the husband is not allowed to consummate the marriage with a young wife unless she becomes physically able to have sexual intercourse.
What you have read or heard in the news that a girl died when giving birth because of bleeding, and she was only nine or ten years old, then this news may be true or false. If we presume that it is true, then death due to birth happens to adult and old women. No rational person would say that this necessitates not marrying women, or not having children.
In conclusion, our advice to you is to respect the limits of the wise Sharee’ah of your Lord, and not look to the West or others to find the truth with them. Their societies include backwardness, evil, tribulation, which only Allah knows. They have homosexuality, family and social disintegration, and debauchery; all this despite their claim that they are civilized and developed.
Allah knows best.