An annulment is a legal procedure that retroactively states that a marriage or civil union never existed. It effectively dissolves the marriage or civil union and returns both parties to their prior single status. In most cases, an annulment is granted because the marriage was not legal in the first place, though grounds for annulment vary in different states. A common misconception is that short marriages can be automatically annulled, while time is actually not a factor in determining grounds for annulment. Annulment is an option only in the following situations:
One party entered into the marriage under duress, threat, or force
One party withheld information that may have prevented the marriage, such as impotence, sterility, criminal history, sexually transmitted diseases, etc.
One party entered the union purely for the purpose of obtaining a permanent residence visa (green card) and kept this information from the other party
For the most part, though, the above criteria are what most states look to when determining grounds for annulment. If you are seeking an annulment, be sure to understand the laws of your state of residence before proceeding.
In the case of same-sex civil unions, annulment laws follow the same guidelines for these unions as they do with traditional marriages, for the most part. However, some states do not recognize same-sex unions, and therefore will not grant annulments to such unions. Since annulments (as well as divorces) are typically granted in the state of residence, be sure to know your state's same-sex union laws. In states where civil unions or same-sex marriages are recognized, annulments can be granted under stringent circumstances, following traditional marriage annulment laws typically to the letter.
Religious annulments, such as those granted by the Catholic church, hold no legal bearing and have completely different rules and guidelines. A religious annulment may be granted in a case where a legal annulment is not, and vice-versa. If you are seeking a religious annulment, you must speak to the proper authorities in your particular area of worship and go through the particular channels as determined by them.