If he asks her to say “I accept So and so as a husband before Allaah”, does she become his wife?
Question

My problem is that I got to know someone on the Net and we loved one another, but after that he asked me to marry him so that our conversing would not be haraam, and the way to do that was that I would say: “I, So and so, accept So and so as a husband before Allaah.” After that he asked me to take off my hijab in front of him but I refused because I was not convinced about this marriage with regard to its conditions. He said: There was acceptance and the mahr (dowry). I said: I don’t want it and there were no witnesses. He said: There were witnesses; but I did not see them myself. He insisted that it is a valid marriage but I am not convinced of that. I hope to find out from you whether I am regarded as his wife or not. I do not want to fall into haraam by speaking to him.
Praise be to Allah.This is not a marriage, rather it is a deviation and calling it a marriage is lying and deceit. What you must do is cut off ties with this man who is toying with the rulings of Allaah, and regarding as permissible that which Allaah has forbidden. His claim that this is a marriage means that he regards it as permissible to do with you what a man does with his wife. Wouldn’t any adulterer on the face of the earth be able to do such a marriage with the immoral adulteress who is his partner in sin, so that she would be his wife and they would not be committing adultery?!  
We are afraid that he may gradually make you take off your hijab in front of him or do something even worse, and take a picture of you and try to threaten you with this picture. It is easy nowadays to combine pictures with one another and threaten you using these pictures if you do not do what he wants. Many such cases have happened. See the answer to question no. 91868 and learn a lesson from it. 
How many heedless women have been gradually tricked in such ways by treacherous “wolves” until they lost their honour and chastity, then these men leave them, forgetting about the so-called marriage, and there is no maintenance and no rights, not even a divorce! 
In a valid marriage it is essential that the woman’s guardian be present, along with two witnesses of good character from among the Muslims. Any marriage that is conducted without a guardian is invalid, and a woman who gives herself in marriage without her guardian is a zaaniyah (adulteress), as Abu Hurayrah (may Allaah be pleased with him) said: “The one who gives herself in marriage is a zaaniyah.” Narrated by al-Daaraqutni.  
The Prophet (peace and blessings of Allaah be upon him) said: “There is no marriage except with a guardian and two witnesses.” Narrated by al-Bayhaqi, classed as saheeh by al-Albaani in Saheeh al-Jaami’ no. 7557. 
And he (peace and blessings of Allaah be upon him) said: “Any woman who gives herself in marriage without the permission of her guardian, her marriage is invalid, her marriage is invalid, her marriage is invalid.” Narrated by Ahmad (24417), Abu Dawood (2083) and al-Tirmidhi (1102); classed as saheeh by al-Albaani in Saheeh al-Jaami’ (2709). 
Its seems to us from your question that you fear Allaah, may He be exalted, and you do not want to fall into haraam and be exposed to the wrath of the Almighty. Hence we advise you strongly to forget about this man and to cut off all ties with him, via the internet or otherwise, for every word, smile and moment of desire will be recorded against you, and tomorrow you will be questioned about it before your Lord. So hasten to repent so that these sins will be erased. We ask Allaah to forgive you and to divert you away from this evildoer, and to protect the daughters of the Muslims. 
See also questions no. 21933 and 84089. 
And Allaah knows best.