If the loan you are asking about is a goodly loan that is to be paid off without paying interest, then there is nothing wrong with you taking it, and may Allaah help you to pay it off. The Prophet (peace and blessings of Allaah be upon him) said: “There are three who are promised the help of Allaah: the mujaahid who is striving for the sake of Allaah, the slave who has a contract of manumission which he intends to pay off, and the one who wants to get married with the intention of remaining chaste.” Narrated by al-Tirmidhi (1655), al-Nasaa’i (3120); classed as hasan by al-Albaani in Saheeh al-Tirmidhi.
And the Prophet (peace and blessings of Allaah be upon him) said: “Whoever takes people’s wealth intending to pay it off, Allaah will help him to pay it off, but whoever takes it intending to use it up (without repaying), Allaah will destroy him. “
Narrated by al-Bukhaari (2387).
But if it is a riba-based loan, in which repayment with interest is required, then it is not permissible for you to take it, whether for the purpose of marriage or anything else, because of the stern warnings which have been issued against riba. For more information please see question no. 6847 and 9054.
How can you start your married life with something haraam? How can you receive divine support when the one who engages in riba is threatened with a curse and is subject to a declaration of war from Allaah and His Messenger?
What you have to do is to be patient and seek reward from Allaah, and wait for a way out granted by Allaah; pursue the means that lead to halaal provision, and do not pay attention to what people say.
One of the means of earning halaal provision is to buy something – a car or a house, for example – by instalments, then sell it for cash to someone other than the person from whom you bought it, even if it is less than the price to be paid by instalments, so that you will have some money available. This is called tawarruq, and is permissible according to the majority of scholars.
See the answer to question no. 45042.
You made a mistake by forming a relationship with this girl, no matter how much you say that it was free of sin, because even if we assume that this is the case, it still harms the girl because of keeping her hanging for such a long time, when in fact you do not have the means to afford marriage. We ask Allaah to forgive you and to bless you by His Bounty with that which will enable you to obey and please Him.
The Standing Committee for issuing fatwas was asked about a man who did the marriage contract but could not afford the mahr; can he take a loan from a riba-based bank?
It is not permissible for you to take this loan, and what you have mentioned about your need for the mahr is not an excuse for taking a riba-based loan from the bank or elsewhere. You have to fear Allaah, for “whosoever fears Allaah and keeps his duty to Him, He will make a way for him to get out (from every difficulty). And He will provide him from (sources) he never could imagine. And whosoever puts his trust in Allaah, then He will suffice him. Verily, Allaah will accomplish his purpose. Indeed Allaah has set a measure for all things” [al-Talaaq 65:2-3]. We ask Allaah to make things easy for you, to relieve your distress and grant you sufficient halaal provision so that you have no need of anything haraam.
End quote from Fataawa al-Lajnah al-Daa’imah (13/387)
And Allaah knows best.