All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.
If the customary marriage fulfils the conditions and pillars of a correct marriage contract, such as the consent of the guardian, the presence of two witnesses, the proposal and acceptance (between the husband and the wife), and making the marriage public, then it is a correct marriage contract. The only difference is that this marriage is not authenticated in court, and it is permissible (acceptable). But we nonetheless do not advise it because of the lack of the fear of Allaah and piety in the era in which we live.
The secret marriage, however, which takes place between a man and a woman without fulfilling the above conditions, is not considered a legal marriage, rather it is an illegal relationship for which the man and the woman concerned are sinful.
Allaah Knows best.