Yes, same sex marriage are legal in the US and are considered legitimate for immigration and green card cases. A problem arises, however, when your spouse resides in a country where same sex marriage is illegal, or where homosexuality is illegal or strongly ostracized. In such places you will not be able to marry your partner, and you may be forced to keep your relationship so secret that you cannot even prove a fiancé relationship to US immigration authorities. A green card marriage lawyer NYC can help you prove your relationship.