Under the Immigration and Nationality Act (INA), a fiance visa holder has 90 days upon entry with a fiance visa to marry the US citizen petitioner before the visa expires. That 90 day period often serves as a test period for couples who have been physically separated, sometimes for months, and have only been corresponding via email and instant messenger, or the phone to keep contact with each other. When reality sets in and the couple realizes that they really don't want to get married after living together a while or just one or both has changed their mind, the fiance visa holder has usually has only one immigration option - to return to their home country.
I often hear from couples who have met and married but do not realize that the immigrant spouse entered on a fiance visa, while engaged to another person, before meeting their current spouse. If that situation occurs, the immigrant has to consular process, and is ineligible for adjustment of status (the green card application process in the U.S.). The immigrant spouse is only allowed to apply for a green card in the U.S. without having to leave the U.S. and visit a consulate abroad if the immigrant marries the same person he or she was engaged to and who sponsored the immigrant originally for a fiance visa.
The problem that's created by this situation is mainly how difficult it could be for the immigrant to be able to re-enter the U.S. legally with another visa even if s/he is now married to a U.S. citizen (not the fiance visa petitioner). Usually, immigrant spouses come to me after many months of unlawful presence, not knowing the consequences of staying in the U.S. beyond the 90 days granted on the fiance visa entry. Many immigrant spouses face the three and ten year re-entry bars to admission and will have to apply for a waiver of the bar but only by showing that their new US citizen spouse will suffer "extreme hardship" if the immigrant spouse is not allowed to re-enter the U.S. So, many times, it's what we would call a "catch 22". The only way to obtain legal status and a green card (permanent residency) is to leave the U.S. to interview for it at a consulate abroad but the very act of leaving the U.S. may trigger one of the unlawful presence bars.
It's very important to document a case of extreme hardship if you can before the immigrant leaves the U.S., if a waiver is going to be attempted. In many circumstances, the risk of not being able to re-enter the U.S. far outweighs the potential benefit of a green card and many spouses wait to file until more time and hardship has developed, despite their unlawful presence in the U.S.
