Does shaking hands with his fiancée’s mother with desire mean that he is not allowed to marry her daughter?
Question

                If one merely touched his mother in law with lust (my hand may have touched her hand or her hand may have touched mine and experienced an erection), does nikah with her daughter become haram?

I need urgent advice in this matter? I am a young man and an immoral person. I have unclean thoughts and frequent erections. Just looking at a woman or if a woman merely touches me by mistake, I have an erection! .
Praise be to Allah.With regard to what you have mentioned about 
    looking at your fiancée’s mother with desire, this does not mean that you are not allowed to marry her daughter, because what would prevent you 
    from marrying her (the daughter) is your marrying her mother and consummating the marriage with her. The evidence for that is the verse in which 
    Allaah says (interpretation of the meaning): 
“Forbidden to you (for marriage) are: … your stepdaughters under your guardianship, born of 
    your wives to whom you have gone in — but there is no sin on you if you have not gone in them (to marry their daughters),”
    [al-Nisa’ 4:23]
 But I advise you to fear Allaah and not take 
    the matter of looking lightly, for it is a serious matter, and the door to endless evil. The Prophet  (peace and blessings of Allaah be upon 
    him) said: “Do not follow one glance with another, for you are allowed the first but not the second.” (Narrated by al-Tirmidhi, 277; Abu Dawood, 
    2147; Ahmad, 1373 – from ‘Ali, may Allaah be pleased with him).
 What you have mentioned about feeling desire 
    whenever you look, is because your mind is constantly preoccupied with sex. Undoubtedly the corrupt media play a great role in that, because their 
    aim is to occupy people’s minds with such things by spreading pornographic movies and pictures, and love stories. These are things that the Muslim 
    should keep away from.
 There is no reason why a person should not 
    satisfy his desires in the way in which Allaah has permitted, but for a man to live with no other thought than how to satisfy his desire, this is 
    something which every wise person should refrain from.
 You should realize that no one will live in 
    this world, rather this is the abode of striving. Man is like a stranger in this world. Think about the words of the Prophet  (peace and 
    blessings of Allaah be upon him) to Ibn ‘Umar – who was a young man – “Be in this world as if you are a stranger or just passing through.”
 Set your aim high and do not be content with 
    anything but the highest and noblest of things. The highest of things is the pleasure of Allaah and entering Paradise. If you set your aim high, 
    you will find that you are a new person, and the things you are suffering will disappear.
 I advise you to obey the command of the 
    Prophet  (peace and blessings of Allaah be upon him): “O young men, whoever among you can afford it, let him get married, and whoever cannot, 
    then let him fast, for it will be a shield for him.”
 (Narrated by al-Bukhaari, 5065; Muslim, 
    1400, from ‘Abd-Allaah ibn Mas’ood, may Allaah be pleased with him).
 Finally, I advise you to make du’aa’ and 
    turn to Allaah, for He will not let down the one who calls upon Him in sincerity and seriousness. Strive to control yourself, and keep away from 
    things that provoke your desire. Fill your heart with the love of Allaah and longing to meet Him.
 May Allaah bless us with beneficial 
    knowledge, righteous deeds, humble hearts and answered prayers. May Allaah bless our Prophet Muhammad and all his family and companions.
 And Allaah knows best.