Sheikh Attiyah Saqr – May Allah have mercy on him- says in his book: the best sayings in Fatwas, and rulings:
The customary marriage is the name given to marriages which do not have an official written document: and they are of two types: one that complies with the rulings and pillars of marriage, and one that does not comply with the rulings.
The first type is a legally correct agreement and it is permissible for enjoyment, and the rights are set for the offspring of the marriage, and likewise for inheritance. This type of system was the most prevalent before newer systems were developed to document these contracts.
As for the second type, it is divided further into two types. One type is done with the agreement of both sides and without the knowledge of any of the witnesses or anyone else. The other type is that the contract is for a certain period of time like a month or a year. Both types are not accepted/ wrong according to all the schools of thought.
As previously stated, the first type of marriage is the only one which is correct according to Islamic Law. This type of marriage permits sexual relationships, but it may have consequences which could lead to forbidden acts such as:
1. It may be against the wishes of the guardian or elders, and obedience to guardians is compulsory in everything which is not considered a sin and is good for us, as Allah SWA says in the Quraan: (O you who believe! Obey Allah, and obey the Messenger, and those charged with authority among you)- surah Nisa ayah 59
2. Due to it not being a documented contract it may cause the rights of the parties to be lost, such as inheritance which cannot be confirmed without documented evidence, the right to divorce may also be lost, and it is not correct for the wife to marry another individual without being divorced by the husband, and he may hold on to her and refuse to divorce her.
And for this reason and many others, the customary marriage without a legal document as evidence is not allowed by Islamic law even with a valid contract. It might be that something is correct but also forbidden, for example the one who prays in a stolen shirt, his prayer is correct but it is forbidden because he stole clothing to cover his body with, in order to make his prayer correct. Likewise one who performs pilgrimage using stolen money, then the obligation is waived, and alongside that he has committed a grave sin because he stole.