Question Summary:
What advice would you give to a married man who does not want children? Is this islamically wrong?

Question Detail:

What advice would you give to a married man who does not want children? Is this islamically wrong?
Should a married couple also be legally married according to their country of residence?

Answer :

In the name of Allah, Most Gracious, Most Merciful
Assalaamu `alaykum waRahmatullahi Wabarakatuh
1) The inception of marriage for mankind on Earth was to maintain the human race through procreation. Allah Ta'ala provided the male with fertilizing organs and the female with a receptive womb for this very reason. Allah Ta'ala says:
والله جعل لكم من أنفسكم أزواجا وجعل لكم من أزواجكم بنين وحفدة ورزقكم من الطيبات
"And Allah has made for you mates of your own nature, and made for you out of them sons and daughters, and grandchildren (and provide for you sustenance of the best)." (16:72)
Thus it can be said that marriage creates families consisting of children and wives. Families create society and the progress of a society means the progress of a nation. Sons and daughters go on to produce new families and a lineage is kept intact. The right of inheritance to the closest relations is also created. If this practice is abandoned, society and civilization would vanish. (Islamic Principles on Family Planning, P. 5-10, Qasmi Publications) 
Rasulullah (Sallallahu Alayhi Wa Sallam) also encouraged to have many children:
تزوجوا الودود الولود فإنى مكاثر بكم الأمم
"Marry affectionate and fertile women, for verily I will take pride through you over other nations." (Sunan Abi Dawud, Vol. 1, P. 280, HM Saeed)
Children are a means of dua and intercession on behalf of their parents:
 عن أبي هريرة : أن رسول الله صلى الله عليه و سلم قال ( إذا مات الإنسان انقطع عنه عمله إلا من ثلاثة إلا من صدقة جارية أو علم ينتفع به أو ولد صالح يدعو له )
"Rasulullah (Sallallahu Alayhi Wa Sallam) said: When a human being passes away, nothing would be of any benefit to him except three things: a continuous charity, some useful knowledge he has left behind, and a child who may pray for him." (Sahih Muslim, Vol. 2, P. 41, Rashidiyya)
It is clear from the above that not wanting children is against the spirit of Islam.
2) Nikah will suffice as the legal marriage in Islam. However, all countries have their own laws in the matter of civil marriage. Scholars in your locality should be consulted with regards to this matter.
And Allah knows best
Wassalam
Ml. Asif Umar,
Student Darul Iftaa
Checked and Approved by:
Mufti Ebrahim Desai
Darul Iftaa, Madrassah In'aamiyyah