What if someone chooses to remain single both in this world and the hereafter?

Firstly:

This question is purely theoretical, as it will not happen on
the Day of Resurrection, because the revelation tells us that there will be
no single people in Paradise.

It was narrated that Abu Hurayrah said: Abu’l-Qaasim
(blessings and peace of Allah be upon him) said: “and there will be no one
in Paradise who is unmarried.” Narrated by Muslim (2834).

Marriage is one of the delights of the people of Paradise,
and the people of Paradise will have no delights except what they themselves
desire, not what they dislike or do not want.

Allah, may He be exalted, says (interpretation of the
meaning):

“Therein you
shall have (all) that your inner-selves desire, and therein you shall have
(all) for which you ask for”

[Fussilat 41:31]

“(there will
be) therein all that the ones inner-selves could desire, all that the eyes
could delight in, and you will abide therein forever”

[az-Zukhruf 43:71].

From this it is clear that as they will all be blessed with
marriage, they will inevitably long for it, and that none of the people of
Paradise will refuse marriage.

Secondly: 

In this world a person may have no desire for a wife, for one
of the following reasons – none of which will be applicable in the
hereafter:

1.

He has not been given any desire for women, because he has
concerns and preoccupations, either religious or worldly, that weaken him
and overcome his desire for women, so he is distracted from them. This is
something that will not happen in Paradise, because in Paradise there is no
worry, fatigue or accountability, as is well known.

Allah, may He be exalted, says (interpretation of the
meaning):

“No sense of
fatigue shall touch them, nor shall they (ever) be asked to leave it”

[al-Hijr 15:48]. 

2.

He is suffering from a psychological or physical problem
which leads to him having no desire or inclination towards women. The people
of Paradise will not suffer from any sickness or problem; rather they will
be given strength for intimacy with women that is greater than that of
people in this world. 

It was narrated from Zayd ibn Arqam that he said: The
Messenger of Allah (blessings and peace of Allah be upon him) said: “A man
of the people of Paradise will be given the strength of one hundred men with
regard to eating, drinking, desire and intercourse.” Narrated by Ahmad
(32/65 and 32/19); classed as saheeh by the commentators on al-Musnad,
and by al-Albaani in Saheeh al-Jaami‘ as-Sagheer (1/335). 

3.

He believes that marriage is a shortcoming and that celibacy
is perfection, as in the case of some of those who claim to be Sufis and
ascetics. Hence he turns away from marriage and does not want to get
married. 

This is ignorance and a lack of knowledge, and the people of
Paradise are far above that, because in fact having desire for and an
inclination towards women, and the strength for intimacy with them, is part
of praiseworthy human perfection. Hence the Prophets, who are the elite of
mankind, used to get married. 

Allah, may He be exalted, says (interpretation of the
meaning):

“And indeed
We sent Messengers before you (O Muhammad (blessings and peace of Allah be
upon him)), and made for them wives and offspring”

[ar-Ra‘d 13:38]. 

Ibn al-Jawzi (may Allah have mercy on him) said: 

Having a lot of wives and concubines is praiseworthy, not
blameworthy. The Prophet (blessings and peace of Allah be upon him) had
wives and concubines. 

Seeking to get married in order to have children is a kind of
worship, and if the aim is to seek pleasure, that is permissible, and may
also be a kind of worship, as it is a means of keeping oneself and one’s
wife chaste, and so on. 

Moosa (peace be upon him) spent ten years of his blessed life
to pay the dowry for the daughter of Shu‘ayb; were it not that marriage is
one of the best of things, the Prophets would not have spent so much time on
it. End quote.

Sayd al-Khaatir
(p. 64-65). 

Turning away from marriage and refraining from it when one
has the ability to get married is something blameworthy that is contrary to
Islamic teaching. 

It was narrated from Anas ibn Maalik (may Allah be pleased
with him) that he said: Three men came to the houses of the wives of the
Prophet (blessings and peace of Allah be upon him), asking about the worship
of the Prophet (blessings and peace of Allah be upon him). When they were
told about it, it was as if they thought it was too little. They said: Who
are we in comparison to the Prophet (blessings and peace of Allah be upon
him)? He has been forgiven his previous and future sins. One of them said:
As for me, I shall pray all night for ever. Another said: I shall fast all
my life and never not fast. And the last one said: I shall keep away from
women and never get married. The Messenger of Allah (blessings and peace of
Allah be upon him) came to them and said: Are you the people who said such
and such? By Allah, I am the one who fears Allah the most among you and am
the most pious among you, but I fast and do not fast, I pray and I sleep,
and I marry women. Whoever turns away from my Sunnah is not of me.”

Narrated by al-Bukhaari (5063) and Muslim (1401). 

Conclusion: this question is based on comparing the life of
the people of Paradise to the life of the people of this world, and
comparing the perfect desires of the people of Paradise to the desires of
the people of this world, which may be affected by shortcomings and
ignorance. Hence this is a false analogy, because it is comparing different
things; it is comparing the perfect life with imperfect life. 

Allah, may He be exalted, says (interpretation of the
meaning):

“The life of
this world is nothing but a distraction and play. Verily the life of the
Hereafter is the only true life, if only they knew”

[al-‘Ankaboot 29:64]. 

Shaykh ‘Abd ar-Rahmaan as-Sa ‘di (may Allah have mercy on
him) said: 

As for the hereafter, it is the abode of “life”, that
is, the perfect life, one of the characteristics of which is that people
there will be physically strong, because their bodies and strength will be
created for life, and everything will be available there that makes life
perfect and brings pleasure and joy and fulfilment of physical desires, by
way of food, drink, spouses and so on, such as no eye has seen, no ear has
heard, nor has it entered the mind of man.

End quote. 

Tafseer as-Sa‘di
(p. 635) 

And Allah knows best.