Illegal marriage is simply marriage that does not conform to the legal restrictions of marriage present in a specific area. If a person is married in an area that allows a certain type of marriage but then moves to or visits an area where that type of marriage is illegal, the rights of that person may not be upheld. While many restrictions on marriage concern the types of people who may become married, motivations for marriage can also be made illegal. Given that what constitutes an illegal marriage differs depending on area, it is important to look into the laws specific to the state in which the marriage is to occur.
In many areas, a marriage may be illegal if one or more of the parties belong to a specific group or if a participant is already married. It is common for underage marriage to be illegal whether one or both partners are underage. Gender is also a large factor in determining if a marriage is an illegal marriage, as many areas hold that marriages between two members of the same biological sex are illegal. In certain places, two people who are getting married must be of the same religion or of the same race. There have been many other restrictions on marriage historically, and it is likely that many more will evolve.
One interesting complication to illegal marriage regulations is that sometimes the reason a person decides to marry can make the marriage either legal or illegal. For example, a green card marriage, in which a citizen of the United States marries a citizen of another country in order to provide that alien with a green card, is often considered illegal marriage. Problematically, it is not often possible to determine which marriages are fraudulent and which are entered into for love. As such, it is usually necessary for a couple to remain married for a number of years before the foreign citizen can get United States citizenship.
It is possible for a certain type of marriage to be illegal in one sense, but permissible in another. Religious marriages need not always line up with state marriages, although religious marriages may not provide the same legal benefits that state marriages can. Even if marriage is not legal for a couple, it is often possible for people to enter into other types of contracts that resemble marriage but do not go by the same name. Also, even if an appropriate agreement cannot be obtained, many areas do not actively prevent those people who are not allowed to marry from engaging in long-term relationships with one another.