Firstly: 

The believer must submit to the rulings and decrees of Allaah. Allaah says (interpretation of the meaning): 

“The only saying of the faithful believers, when they are called to Allaah (His Words, the Qur’aan) and His Messenger, to judge between them, is that they say: ‘We hear and we obey.’ And such are the successful (who will live forever in Paradise)”

[al-Noor 24:51]

If the believer is confused about anything in the rulings of Allaah, and he does not know what it means or what the reason behind it is, then he must say as those who are firmly grounded in knowledge say: 

“We believe in it; the whole of it (clear and unclear Verses) are from our Lord”

[Aal ‘Imraan 3:7 – interpretation of the meaning]

It is not permissible for the believer to say that one of the rulings of Allaah is not fair; exalted be Allaah far above that. Allaah says (interpretation of the meaning): 

“And your Lord is not at all unjust to (His) slaves”

[Fussilat 41:46]

There is no rule that is better than the rule of Allaah. Allaah says (interpretation of the meaning): 

“Is not Allaah the Best of judges?”

[al-Teen 95:8]

“Do they then seek the judgement of (the days of) Ignorance? And who is better in judgement than Allaah for a people who have firm Faith?”

[al-Maa’idah 5:50]

Secondly: 

In this question there are two mistakes and a misunderstanding. The first mistake is where the questioner says that in Paradise a man will have seventy hoor ‘iyn. What is proven in the saheeh Sunnah is that the martyr (shaheed) will have seventy-two hoor ‘iyn. The least of the people of Paradise will have two wives, and some will have more than that. 

It was narrated that al-Miqdaam ibn Ma’di Yakrib said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “The martyr has six blessings with Allaah: he will be forgiven from the first drop of blood shed; he will be shown his place in Paradise; he will be protected from the torment of the grave; he will be safe from the greater terror; a crown of dignity will be placed on his head, one ruby of which is better than this world and everything in it; he will be married to seventy-two wives from al-hoor al-‘iyn; and he will intercede for seventy of his relatives.” 

Narrated by al-Tirmidhi, 1663; Ibn Maajah, 2799; classed as saheeh by al-Albaani in Saheeh al-Tirmidhi. 

And more than that has been narrated. Abu Na’eem narrated in Sifat al-Jannah from Abu Hurayrah (may Allaah be pleased with him) that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “A man will go to one hundred virgins in one day – i.e., in Paradise.” Classed as saheeh by al-Albaani in al-Silsilah al-Saheehah, 367. 

It was narrated from Abu Sa’eed al-Khudri that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “The lowest of the people of Paradise in status will be a man whose face Allaah will turn away from the Fire towards Paradise, and make a shady tree appear before him. …  Then he will enter his house and his two wives from among al-hoor al-‘iyn will enter after him. They will say: ‘Praise be to Allah Who has created you for us and us for you.’ And he will say: ‘No one has been given the like of that which I have been given.’” Narrated by Muslim, 188. 

Al-Haafiz said:  

The apparent meaning is that the least that will be given to each of them will be two wives. 

Thirdly: 

The second mistake is the questioner’s saying that al-hoor al-‘iyn will serve the man. This is not correct, rather the ones who will serve the people of Paradise will be boys of everlasting youth. Allaah says (interpretation of the meaning): 

“And round about them will (serve) boys of everlasting youth. If you see them, you would think them scattered pearls”

[al-Insaan 76:19]

Al-hoor al-‘iyn will be wives of the men in Paradise, in addition to their wives from among the people of this world. Allaah says (interpretation of the meaning): 

“So (it will be). And We shall marry them to Hoor (fair females) with wide lovely eyes”

[al-Dukhaan 44:54] 

“They will recline (with ease) on thrones arranged in ranks. And We shall marry them to Hoor (fair females) with wide lovely eyes”

[al-Toor 52:20]

Fourthly: 

The misunderstanding is when the questioner says: “This to me does not seem like a fair concept for the women, to share their husbands in this way.” 

Fairness or justice is to be found in the rulings of sharee’ah, not in what a person thinks who does not know sharee’ah and its rulings, let alone the reasons behind them. 

The sister who is asking this question thinks that the jealousy in her heart, and what it leads to of depression and sorrow will remain with her in Paradise, but this is not correct, and this is why there is this misunderstanding in her question.  

Allaah says (interpretation of the meaning): 

“And We shall remove from their breasts any (mutual) hatred or sense of injury (which they had, if at all, in the life of this world); rivers flowing under them, and they will say: ‘All the praises and thanks be to Allaah, Who has guided us to this, and never could we have found guidance, were it not that Allaah had guided us! Indeed, the Messengers of our Lord did come with the truth.’ And it will be cried out to them: ‘This is the Paradise which you have inherited for what you used to do’”

[al-A’raaf 7:43]

There is nothing but joy and happiness in Paradise; there is no room for hatred and rancour in the hearts of the people of Paradise. Al-hoor al-‘iyn are something which Allaah has created to honour the people of Paradise and to increase their delight. Moreover a man will be given the strength of one hundred men for intercourse, so the large number will not have any effect on a woman, and her feelings towards her co-wives and her husband’s concubines will not be like her feelings in this world. 

It was narrated that Zayd ibn Arqam said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said to me: “A man in Paradise will be given the strength of one hundred men in eating, drinking, desire and intercourse.” A Jewish man said: “The one who eats and drinks will need to relieve himself.” The Messenger of Allaah (peace and blessings of Allaah be upon him) said to him, “The way in which one of them will relieve himself will be by means of sweat which will come out through his pores, and thus the food in his stomach will be digested.” 

“Do they then seek the judgement of (the days of) Ignorance? And who is better in judgement than Allaah for a people who have firm Faith?”

[al-Maa’idah 5:50 – interpretation of the meaning]

Narrated by Ahmad, 18827; classed as saheeh by Ibn Hibbaan, 16/443; and by Shaykh al-Albaani in Saheeh al-Jaami’, 1627. 

It was narrated from Anas that the Prophet (peace and blessings of Allaah be upon him) said: “The believer in Paradise will be given such-and-such strength for intercourse.” It was said, “O Messenger of Allaah, will he really be able to do that?” He said, “He will be given the strength of a hundred (men).” 

Narrated by al-Tirmidhi, 2536; classed as saheeh by Ibn Hibbaan, 16/413; and by Shaykh al-Albaani in Saheeh al-Jaami’, 8106. 

And Allaah knows best.