Is it permissible to take out an interest-based loan from a bank for the purpose of marriage, given that there is currently an unlawful relationship?

Borrowing with usury is a grave sin that is only permissible in a dire necessity, which is when one is overwhelmingly likely to fall into ruin, or an unbearable hardship, or inability to secure the minimum standard of living for the poor. Fear of falling into what is forbidden due to delayed marriage is not a necessity that permits borrowing with usury, because marriage is not the only way to avoid what is forbidden. One should cut off the causes of falling into what is forbidden, such as mixing (of genders), looking (at what is unlawful), seclusion, and heedlessness of the remembrance of Allah, and repent to Allah and end the relationship.