It depends on a number of facts, including how long you've been married, and how long ago you found out. Annulments are more complicated than dissolutions (divorces) so you should speak to an experienced family law attorney.
As the previous answer indicates, length of the marriage will be important, as will the question of how you learned about it.
The basis of an annulment is that the person seeking one must prove that he or she was induced to marry on the basis of a fraud (he said he was straight, but actually is gay). You should contact a local divorce attorney to see if your case will work or whether you will need to file for a divorce.
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