One specific area where this has been a concern is in the case of married couples where one of the spouses was transgender. Because of the so-called Defense of Marriage Act (DOMA), the federal government can not provide a benefit based on a same-sex marriage. For transgender people, determining what their sex is for purposes of marriage recognition can be legally frustrating and confusing enterprise. The memorandum makes it clear that if a transgender couple has been recognized as a different-sex couple for the purpose of state law, then they will also be considered as such for immigration purposes, thus avoiding any DOMA-related problems.
From the press relase from Immigration Equality:
You can read the full 7-page Memorandum for yourself below.
U.S. Immigration Trans Policy