Nikah Siri is marriage performed just according to Islamic Law (Sharia), without registered according to stipulation of Marriage Law in Indonesia. It is usually done secretly, so just known by certain people. Even it is enough if only presented by the couple, wali nikah (marriage guardian), witness, and some close families. Because of secret, so no celebration, no party.
There are some reasons why someone commit nikah siri. From financial reason until adultery reason. From polygamy reason until job reason.
For the poor, they choose to commit nikah siri because of no choice. Marriage frankly (according to State Law) means party, and party is so expensive. And they have no money. So save method to marry is nikah siri.
Sometimes the young commit nikah siri because they are still student and have not worked. Also because to prevent from adulterous behavior. As in Jogjakarta. As a student city, there are boys and girls, studying in some universities. They come from all region of Indonesia, even from other countries. And most of them are muslim. As wanderer, young, of course there are so many temptation. So to protect and prevent from adultery (zina), they commit nikah siri. And after graduate, some of them continue their marriage by registration, and the others choose to separate (divorce).
Sometimes nikah siri is also done because the bride-groom has had wife before. So for 2nd marriage, he commit nikah siri. Mereover if he is a civil servant who prohibited to commit polygamy. If broken the rule, he will get punishment.
Except those above, there are some jobs that require marital status single for employee for certain period.
Because of no registration, nikah siri has negative effects. It is about children resulted from nikah siri. Although nikah siri is valid according to Sharia, but no marriage according to State Law. Because no registration means no document of marriage. It is same with no proof of marriage. It is influenced to legal status of children. According to State Law, they are born out of wedlock. Illegitimate children. Consequently, they just have legal connection with their mother, not their father.
Other effect is if divorce happen. Wife can not sue to livelihood (mut’ah) if she divorced by her husband. She also can not inherite from her husband if her husband is dead, and her children can not too.
That’s why nikah siri is always be controversy. Because of no legal certainty. No legal protection from State to women and children.