Why are some
    things forbidden in this world when they are permitted in Paradise?
Question

                Before I get to my question, I would
      like to introduce myself a little. I am a muslim girl in Sweden, who got a question from a
      christian person. I've asked everyone around me and tried to find the answer by looking
      in books, internet, but unfortunately whithout any results.The question was about huriyat
      (several women), according to what I've heard it is several women that the muslim men will
      be "awarded" with,in janna(heaven).I don't know if this information is right,
      but if you could give me a little bit more information about this, I would be thankful.
      The main question is: -Why does Islam encourage something in janna(heaven), which is
      forbidden here on earth?. Whith that he means, that a relationship outside marriage is according to Islam- haram (forbidden), if they(muslim
      men)avoid this on earth, they will be awarded with huriyat in janna. Isn't that very
      strange? Unfortunately a know very little about this subject, I don't know where he got
      this question, but I am sure there is a logical explanation for this question, which I
      hope to find answer to.I would be very grateful, if you could help me with this question,
      thank you for your time!
Praise be to Allah.In His Holy Book, Allaah has mentioned Paradise and what
    He has prepared therein. He has mentioned the attributes of Paradise and of its people in
    numerous places in the Quraan, such as the aayaat (interpretation of the meaning):
Therein will be a running spring.
Therein will be thrones raised high.
And cups set at hand.
And cushions set in rows.
And rich carpets (all) spread out
[al-Ghaashiyah 88:12-16]
But for him who fears the standing before his Lord, there will
    be two Gardens (i.e. in Paradise). 
Then which of the Blessings of your Lord will you both (jinn and men)
    deny?
With spreading branches.
Then which of the Blessings of your Lord will you both (jinn and men)
    deny?
In them (both) will be two springs flowing (free).
Then which of the Blessings of your Lord will you both (jinn and men)
    deny?
In them (both) will be every kind of fruit in pairs.
[al-Rahmaan 55:46-52]
Wherein both will be Qaasiraat ut-Tarf [chaste females
    (wives) restraining their glances, desiring none except - their husbands], with whom no
    man or jinni has had Tamth [sexual intercourse] before them. 
Then which of the Blessings of your Lord will you both (jinn and men)
    deny?
(In beauty) they are like rubies and coral.
[al-Rahmaan 55:56-58]
Hoor (beautiful, fair females) guarded in pavilions
[al-Rahmaan 55:72]
And (there will be) Hoor (fair females) with wide lovely eyes
    (as wives for Al-Muttaqoon  the pious). 
Like unto preserved pearls.
A reward for what they used to do.
[al-Waaqiah 56:22-24]
There are many saheeh ahaadeeth from the Prophet 
 (peace and blessings
    of Allaah be upon him) which describe the women of Paradise and state that they will be
    prepared on the Day of Resurrection for the muttaqoon (pious). Among these
    ahaadeeth is the hadeeth of Abu Hurayrah (may Allaah be pleased with him) who said, the
    Messenger of Allaah 
 (peace and blessings of Allaah be upon him) said:
The first group to enter Paradise will look like the moon on the
    night when it is full, then those who follow them will look like the brightest star in the
    sky; they will not urinate or defecate or spit or blow their noses. Their combs will be of
    gold and their sweat will be musk and their incense-burners will be pearls. Their wives
    will be al-hoor al-iyn. Their appearance will be the same and they will look
    like their father Adam who was sixty cubits tall.
(Saheeh al-Jaami, 2015).
It was also reported that he 
 (peace and blessings of
    Allaah be upon him) said: A tent will be a pearl sixty miles high, in each corner of
    which will be a wife for the believer whom no one else will see. (Saheeh
    al-Jaami, 3357). 
    In these ahaadeeth the women of Paradise are mentioned
    who have been prepared for the men. In His Book, Allaah called them al-Hoor. Al-Hoor
    is the plural of Hooraa . Al-Qurtubi said in al-Ahkaam (17/12):
    (This means) a woman in whose eyes the white is intensely white and the black is
    intensely black. We believe in that with absolute faith uncontaminated with any
    doubts. It is part of the essentials of our belief. For more information see Saheeh
    al-Bukhaari, Kitaab Bad al-Khalq, Baab Sifat al-Jannah; and Saheeh
    Muslim, Abwaab Sifat al-Jannah; and Sifat al-Jannah by
    Abul-Naeem al-Isfahaani, which describes the women of the people of Paradise
    and their beauty.
With regard to the question about why Islam encourages and gives glad
    tidings of things in Paradise which are forbidden in this world, such as men having
    intimate relationships with women outside of the framework of marriage  before we
    answer this question, we must make a serious point, which is that Allaah forbids whatever
    He wills in this world to its people, for He is the Creator and Sovereign of these things,
    so it is not permissible for anyone to object to the ruling of Allaah on the basis of his
    foolish thinking and imperfect understanding. The ruling and the decision of the matter,
    before and after, is only with Allaah, and there is none who can overturn His ruling, may
    He be glorified and exalted.
With regard to the question of why Allaah forbids some things in this
    world, then in the Hereafter will permit them as a reward to those who abstained from them
    (such as wine, zinaa, wearing silk for men, etc.), this is what Allaah wills to give as a
    reward to those who obey Him and patiently strive in this world. Allaah says
    (interpretation of the meaning):
Is there any reward for good other than good?
    [al-Rahmaan 55:60]
With regard to the reason why these things are forbidden, there follow
    a few important points:
Firstly:
We do not necessarily know all the reasons why things are forbidden.
    There are some reasons which we do not know. The basic principle is to accept and submit
    to the texts even if we do not know the reason, because submission is the point of Islam
    which is based on complete obedience to Allaah, may He be exalted.
Secondly:
Some of the reasons for the prohibitions may be clear to us, such as
    the corruptions that result from zinaa, e.g., the mixing of lineages, the spread of fatal
    diseases and so on. When Islam forbids unlawful relationships, the intention is to protect
    peoples lineage and honour, which may not mean anything to the kaafirs and immoral
    people, who jump on one another like donkeys, with a boyfriend having intercourse with his
    girlfriend and people having intercourse with their own relatives and so on, as if they
    are in the jungle. Indeed, they are worse than animals, for some animals refuse to do such
    a thing, but these people do not refuse and do not care. As a result of this promiscuity,
    the ties that bind society disintegrate and it becomes filled with fatal sexual diseases
    which are an indication of the wrath of Allaah towards those who violate His sacred limits
    and make permissible that which is forbidden.
This is unlike the relationship of a man with the Hooris in
    Paradise  which is what has been asked about here  for we may note that in the
    case of a promiscuous woman in this world, her honour is readily available to all and she
    has no religious commitment or shame. She is not bound by a lawful relationship to one
    person. So men and women have intercourse with whomever they want, with no restrictions of
    religion or morality. But the hooris in Paradise are restrained and restricted to
    their husbands to whom Allaah has given them as a reward for their patience in abstaining
    from haraam things in this world. As Allaah says (interpretation of the meanings):
Hoor (beautiful, fair females) guarded in pavilions
[al-Rahmaan 55:72]
with whom no man or jinni has had Tamth [sexual intercourse]
    before them. [al-Rahmaan 55:56]
The hoori is a wife for the man, as Allaah says (interpretation
    of the meaning):
 and We shall marry them to Hooris [female fair ones]
    with wide, lovely eyes [al-Dukhaan 44:54].
And they will be confined to their husbands, and no one else will have
    a share in them.
Thirdly:
Allaah  who has declared that in this world a man is not
    permitted to have more than four wives at one time  is the One Who will bless the
    people of Paradise with whatever He wills of al-hoor al-iyn. So there is no
    contradiction between what is forbidden in this world and in the Hereafter, because the
    rulings of each are different, in accordance with what the Lord wills. Undoubtedly the
    Hereafter is far better and more lasting than this world. Allaah says (interpretation of
    the meaning):
Beautified for men is the love of things they covet; women,
    children, much of gold and silver (wealth), branded beautiful horses, cattle and
    well-tilled land. This is the pleasure of the present worlds life; but Allaah has
    the excellent return (Paradise with flowing rivers) with Him.
Say: Shall I inform you of things far better than those? For
    Al-Muttaqoon (the pious) there are Gardens (Paradise) with their Lord, underneath which
    rivers flow. Therein (is their) eternal (home) and Azwaajun Mutahharatun (purified mates
    or wives). And Allaah will be pleased with them. And Allaah is All-Seer of the (His)
    slaves.
[Aal Imraan 3:14-15]
Fourthly:
This prohibition may be a kind of test from Allaah to His slaves, to
    see whether they will obey these commands and avoid that which He has forbidden. The test
    could not be with regard to something for which people have no inclination and which they
    do not like; it has to be with regard to something to which they are inclined and
    attracted. This testing may include wealth  will a person take from that which is
    permitted and dispose of it in manners which are permitted, and will he fulfil the rights
    which Allaah has over it? The test with regard to women is: will he restrict himself to
    that which Allaah has permitted him of women, and will he lower his gaze and avoid seeking
    pleasure in that which Allaah has forbidden. It is an aspect of Allaahs mercy 
    may He be exalted and glorified  that He does not forbid something to which people
    are inclined without allowing something of that nature and there is much which is
    permitted.
Fifthly:
The rulings (ahkaam) of this world are not like the rulings of
    the Hereafter. The wine of this world causes people to lose their minds, unlike the good
    wine of the Hereafter which does not make people lose their minds or cause headaches or
    stomachaches. What Allaah has prepared for the believers of women on the Day of
    Resurrection as a reward for their obedience is not like zinaa which violates
    peoples honour, confuses lineages, spreads diseases and leads to regret. The women
    of Paradise are pure and good; they will never die or get old, unlike the women of this
    world. Allaah says (interpretation of the meaning):
Verily, We have created them (maidens) of special creation.
And made them virgins.
Loving (their husbands only), (and) of equal age
[al-Waaqiah 56:35]
We ask Allaah to bless us with the best of this world and the next, and
    to help us to be obedient to His commands and to have certain faith in His reward and to
    keep us safe from His punishment. And Allaah knows best.