Here is some information that may be useful to the OP that touches on
marriage, emigration, and immigration.
If the woman is a pre op transsexual and gets married in a state that
allows it, that would be deemed a same sex marriage, if she is post op
and has had her birth certificate amended that would constitute a
mariage between a man and a woman, as such the Defense of Marriage
Act (DOMA) does not apply to post op women.
As regards the point under discussion, marriage is a family law-based
concept and family law is governed by state law, not federal.
While immigration law is federal, the Citizenship and Immigration
Services looks to the law of the state where the marriage was entered
into to determine whether the marriage is valid.
Thus in the states where a woman is able to have her birth certificate
amended to be congruent with her gender after GRS, and she is
permitted by that state to marry, then the federal government will
recognize the marriage.
As an aside the vast majority of states and nations allow the afore-
mentioned to occur.
This has been borne out in re Lovo-Lara, 23 I&N Dec. 746 (BIA 2005).
This was a case before the Board of Immigration Appeals which
adjudicated that post operative women were indeed women for
purposes of immigration and marriage.
Lovo-Lara was a post op transsexual in North Carolina, a state that
allows for the amending of birth certificates following GRS, and
allows marriage as well.
Another relevant case is re Widener, A95-347-685, 2004 WL 2375065
(BIA Sept. 21, 2004). Esperanza Widener was born biologically male in the
Philippines. She had sex reassignment surgery in the Philippines and
received a court order here changing her sex and her name.
The Board of Immigration Appeals also ruled in her favor.
As has been pointed out Idaho, Ohio, Kansas, Tennessee, and Texas,
would preclude a post op woman from legally marrying. Though the
civil case currently going on in Texas involving Nikki Araguz, the
widow of a firefighter may set new precedent.
In terms of countries that allow a post op woman to marry, France,
Germany, Italy, the Netherlands, Portugal, Denmark, Finland, Sweden,
the United Kingdom, Canada, Australia, South Africa, China, Singapore,
Japan, Argentina, Brazil, Cuba, Iran are a few of the countries.
For women in Idaho, Ohio, Kansas, Tennessee, and Texas who are
desirous of marrying, and that are concerned about survivor benifits,
she and her husband should make sure that their wills clearly outline
who they want their assets to go to in the event of their death.