Question Summary:
Should I consider a secret Nikāh?

Question Detail:

i am a realationship with a guy my family knows bt did not agreed we meet and are in contect he is married and his family has no problem can we do secret nikkah for sake of sex

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
You state that you wish to perform a secret Nikāh to fulfill your intimate needs. Nikāh is a sacred institution and lifelong commitment. It is not merely a temporary visa that enables one to fulfill his/her carnal desires. Through the bond of marriage, two individuals come together and make a decision to live their lives together in order to spiritually enhance themselves and attain the love of Allāh Ta’ālā. 
Before you make a haphazard decision, consider a few points. Are you making such a big decision that will affect the remainder of your life based on your emotions? Are you sure that the man in reference is sincere in wanting to keep you as his second wife? What if he is merely using you to fulfill his lustful desires?
In case your relationship with him does not work out, he has a family to return to. You are going against the will and wish of your family in continuing your relationship with him. If the relationship does not work out, you may be left without anyone to support you.
Our advice to you is that you consult your family members and seek their guidance. You have a bright future ahead of you. Make a decision that will please them. They are your elders and have your goodness in mind.  
In the meantime, cut off all relationships with the man. It is impermissible to meet and entertain a relationship with a strange man.  Seek Allāh’s forgiveness for having been engaged in an impermissible relationship. This “love at first sight” is in fact an attack from Shaytan.  You do not know the reality of this man, whether relationship with him will be a source of happiness or not. Control your gaze and refrain from thinking about him.  The more you keep your mind occupied in thinking about him it will create restlessness within your heart.
We make Dua that Allah Ta’ala guide you in making the correct decision. Ameen.
And Allah Ta’āla Knows Best
Abdul Azīm bin Abdur Rahman,
Student Darul Iftaa
U.S.A.
Checked and Approved by,
Mufti Ebrahim Desai.