Marrying a man with a bad past
Question

                In the Name Of God Most Gracious and
      Merciful.
   Salam elkum.
First I would really like to commend you on the substantial information
      you have provided to the public. May God reward you all on your efforts. I do realize that
      every single question cannot be answered directly. However, I have done a lot of research
      on this matter and still I have come up with no answers to my question. What makes it even
      more difficult is the lack of knowledge of Islam from my parents perspective. I was born
      in Canada and one of the very few young teenage girls fighting to learn more about Islam.
      Unfortunatly, there is still a lot I do not know about our religion, yet I work at it
      every day.
    Getting down to my problem, I am 19 years old and just got engaged to a Lebanese Muslim fellow. After getting engaged to this person I found
      out that in the past he has been with other girls and intimate relationships, before
      marriage. Of course, I realize that in our religon that is very wrong. Now I am left with
      the decision of whether to go ahead and get married to this person or not. I personally
      think that I shouldn't be with a person who has done such things while my family says
      forgive and forget... However, I would like your opinion on this matter. Is it ideal and
      acceptable for someone like myself to marry a person of this nature even if it was his
      past.
    Thank you so much for your time . I would appreciate it if you had a
      chance to reply it would fill up so many unanswered questions but of course I fully
      understand if you cannot do so .
      It is very difficult for me to learn more about Islam sometimes books
      don't fully answer my questions. Thanks again kindly for your time .
Praise be to Allah.First of all, may Allaah reward you with good for your kind words. We
    apologize for not answering questions in full. With regard to your question about this man
    who has asked for your hand in marriage, what you should concentrate on is the way he is
    now: is he doing his religious duties, like the five daily prayers, etc., is he keeping
    away from haraam things, has he repented for what he did in the past? Or not? If he is
    keeping his duty towards Allaah, then this is the good commitment to religion that is
    required according to the command of the Prophet 
 (peace and blessings of Allaah be
    upon him): If there comes to you with an offer of marriage (for your daughter,
    sister, etc.) one with whose religious commitment and character you are pleased, then
    marry (your daughter, sister, etc, to) him. If you do not do so, there will be mischief on
    earth and widespread corruption. (Reported by al-Tirmidhi, 1004; classed as
    hasan in Saheeh al-Jaami, 270)
    It is not permitted to bring up the past of a person who
    regrets it and has repented and given up his sin; rather, this should be covered up.
    Whoever covers up his Muslim brother in this world, Allaah will cover him up in the
    Hereafter. (A saheeh hadeeth reported by Imaam Ahmad; Saheeh
    al-Jaami, 6287)
    But if a person is immoral and corrupt, and still has
    relationships from before, then you should never, ever agree to marry him. Allaah says
    (interpretation of the meaning): The adulterer marries not but an adulteress or a
    mushrikah. And the woman who agrees to marry a mushrik or an adulterer, then she is either
    a prostitute or a mushrikah. Such a thing is forbidden to the believers. [al-Noor
    24:3]
Ibn Katheer said, commenting on the phrase Such a thing is forbidden
    to the believers: i.e., (it is forbidden for) a man to marry an immoral woman,
    or for a chaste woman to marry an immoral man. Hence Imaam Ahmad ibn Hanbal said that the
    marriage contract between a chaste man and an immoral woman is invalid, so long as she
    remains like this, unless she is asked to repent and she does so, in which case the
    contract becomes valid, otherwise it is not valid. Similarly, the marriage of a free,
    chaste woman to a promiscuous, immoral man is invalid unless he repents sincerely, because
    Allaah says Such a thing is forbidden to the believers. 
There is no secret about the mischief, misery and problems that result
    from marriage to an immoral man.
In many cases it is very difficult to get a true picture of a person
    and be sure about whether he is chaste or otherwise. But by researching, asking questions,
    consulting people and asking them for advice, whilst taking ones time and asking
    Allaah for help, one can get answers. We ask Allaah to choose the best for you, to help
    you and to guide you. May Allaah bless our Prophet Muhammad.