Why does Islam allow a man to marry up to four women, while women are allowed to marry only one man? What are the reasons behind this?
A man is allowed to marry up to four women for many reasons, benefiting both the man and the women.
Firstly, it may help the women as in the case of the Prophet where he often married poor women or women whose husbands died in battle. This was to help and protect them.
Secondly, as the prophet Abraham (as) married, they may bear the man a child when the first wife cannot. Instead of abandoning a women for her inability to conceive, he marries another and *treats them both with justice)
And lastly, in the case of a man tempted to commit adultery with another women. Instead of this man committing a grave sin, adultury, Allah has given man an ease within his law and allowed man to marry another women. The aim of this is to prevent man and the women from committing a grave sin. Many people look at this law as a selfish addition to men but in fact it is meant as an ease on both men and women; it's not an excuse to 'have fun' with many wives but in fact it is meant to help ease men and women with their way of life.
One rationale that, to me, seems pretty logical and which is (not sure about it since I can't find it any authentic place over the internet at least) narrated from Imam Ali(s.a.).
The gist of it is that if polyandry was to take place how would one identify the actual biological father of a given baby if her mother happened to be engaged in polyandry at the time of conception. Countering the argument, that e.g. suggest DNA test, it will most probably not work here and isn't feasible in most cases. It's like mixing two similar liquids and asking to separate them back into there respective containers.
Now the actual narration, quoted verbatim:
it is said that many women came one day to imam ali peace be upon him asking him : oh ali why did islam allow polygamy (or multiple marriage) for men and didnt allow this for women ,, so imam ali peace be upon him ordered the women to bring small containers and fill them with water , he gave each woman one container and ordered them to pour the water in a big one he put in the middle ,,, then imam ali peace be upon said each one of you fill her container with the same water she poured from it . the women replied how come the water is already mixed and cant be identified(or classified) ,,then imam ali peace be upon him answered then if a woman had more than one husband how can she identify the father of her baby end of story
And therefore polyandry is strictly prohibited it in Islam. After all it's our good that Allah wants.
Because it is necessary under some conditions.
وَاٰتُوا الْيَتَامٰى اَمْوَالَهُمْ وَلَا تَتَبَدَّلُوا الْخَبٖيثَ بِالطَّيِّبِ وَلَا تَاْكُلُوا اَمْوَالَهُمْ اِلٰى اَمْوَالِكُمْ اِنَّهُ كَانَ حُوبًا كَبٖيرًا
And give to the orphans their properties and do not substitute the defective [of your own] for the good [of theirs]. And do not consume their properties into your own. Indeed, that is ever a great sin وَاِنْ خِفْتُمْ اَلَّا تُقْسِطُوا فِى الْيَتَامٰى فَانْكِحُوا مَا طَابَ لَكُمْ مِنَ النِّسَاءِ مَثْنٰى وَثُلٰثَ وَرُبَاعَ فَاِنْ خِفْتُمْ اَلَّا تَعْدِلُوا فَوَاحِدَةً اَوْ مَا مَلَكَتْ اَيْمَانُكُمْ ذٰلِكَ اَدْنٰى اَلَّا تَعُولُوا
And if you fear that you will not deal justly with the orphan girls, 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 or those your right hand possesses. That is more suitable that you may not incline [to injustice].
Nisa 2-3 (4 / 2-3)
As you see in the ayats, Muslims are not allowed to multi-marry any woman. This law is primarily for justly dealing with orphaned women. And, as you see in the ayats, Allah does not suggest or encourage us to marry more than one woman. Allah only shows this as a path for keeping rights of orphan women. If we fear that we won't be able to behave justly, we are not allowed to marry more than one woman.
However, today, some people have changed the meanings of ayats. They generalized the meanings of this ayats to make them fit to their desires. They used these ayats to make themselves harems. And yet they treat their wives as animals instead of protecting their rights, as if they were just child producing machines.
The Quran has show us this multi-marriage formula as a path to solve a problem. Not to make a harem for ourselves. People's evil doings should not dirty Islamic laws.
I also want to add this: is it worse to marry up to 4 women, or is adultery every night with a different woman? Some people oppose to this multi-marriage law simply because it is an Islamic law. But they are remaining blind to those who make adultery with many more that 4 women.
Islam is the only religion that disallows men from marrying more than 4. It is not like marry 4. It is rather like you can marry up to 4 and that to only if you can make justice. The condition of making justice is very hard to achieve. So, it is not that easy. In my knowledge no other religion has anything in written about any limit of marriage. You can search in youtube for Dr. Zakir Naik's videos on marriage.
In that video also you'll get to know that the world has more than 3 times (nearly 4 times) women than men. So, Islam is completely socially balanced. If it is not so then millions of Women will remain single or commit haram.
The simple answer is because that is the role Allah has for women. Her reason for existence is to produce babies - something she can do at most once a year. A man however can create many babies, even in 1 night. If a woman were to have more than one husband, what would she do? Have a child with one one year, another the next?
Allah has thought of everything which is why it is like this and given humanity a guide. A good muslim should not ask 'why' it must be like this but rather work out 'how' i can live my life by the holy Quran.
If you see these stats of wikipedia you'll know that most countries of the world have a greater population of women than men (notice the pink regions). In addition you'll find many gay people there who are not interested in women :) This will cause serious problems to women in finding their husbands. And this will lead them to adultery and other things like that to fulfill their desires. Islam gives the best solutions to these problems. Regards.