Unmarried children beware. 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 you complete the process of getting a green card (U.S. lawful permanent residence).
Why? Because you must not only qualify for the green card when the visa petition is first approved on your behalf, but up until when you are actually granted permanent residence -- in fact, if you come from overseas, up until you actually enter the United States with your immigrant visa. In some cases, depending on your visa category, marrying will completely cancel your application process. In other cases, it will mean you drop into a lower eligibility category, and face years of delays.
Here are some examples of how this could play out:
The marriage issue can be a problem for many young immigrants, who may 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 -- especially if they think that, by getting married, they'll be able to bring their spouse along with them to the United States.
Some 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? But the truth is likely to come out eventually, for example if the child later files a visa petition to bring his or her 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). And committing fraud in order to get a visa (i.e. by concealing one's marriage) can be grounds for removal from the United States.
If you have any questions, or claimed a green card as an unmarried child despite having been married first, see an experienced attorney.