If you are the unmarried son or daughter of a permanent resident or a U.S. citizen, and you plan to immigrate to the United States as that person's beneficiary, you must not marry before completing the process of getting a green card (U.S. lawful permanent residence). Too many young people have gotten married thinking their new spouse could also come to the U.S., and thereby destroyed their own eligibility for the U.S. visa.
U.S. immigration law says you need to not only qualify for the green card when the I-130 petition filed by the U.S. sponsor is first approved on your behalf, but all the way until when you are actually granted permanent residence. In fact, if you come from overseas, you'll need to stay unmarried up until you actually enter the United States with your immigrant visa.
In some situations, depending on your visa category, marrying will completely cancel your application process. In others, it will mean you drop into a lower eligibility category, and face years more of waiting.
Here are some examples of how this could play out:
The marriage issue can be a problem for many young immigrants, who might grow up during the long wait for a visa, and don't want to put their marriage on hold. However, they should understand the consequences of marriage before making a decision.
Some would-be immigrants have attempted to conceal the fact of their wedding. For example, if the child marries after the visa interview but before entering the U.S. with an immigrant visa, how will anyone know about it?
The truth is likely to come out eventually, for example if the child later files an I-130 petition to bring the spouse the United States (which would require submitting a copy of the marriage certificate) or applies to become a U.S. citizen (at which time the immigration authorities will review the person's entire file).
Committing fraud in order to get a visa (such as by concealing one's marriage) can be grounds for removal from the United States.
If you have any questions, or have claimed a green card as an unmarried child despite having been married first, see an experienced attorney.