For taxes the answer is YES!
But then that is under these conditions.
They are considered to be married if they were lawfully married in an American state or a foreign country.
That state or country must have a law that covers the acceptance of the marriage, and authorized the marriage of two people of the same sex.
Ans: The state or country where they were legally married will take precedence in this case and they are covered for recognition of marriage by the IRS even if they are not in the state or country of their legal marriage.
Ans: Whether the married couple are in a state that recognizes their marriage or not the IRS feels that the words husband, wife or spouse or husband and wife, all relate to the couple whether they are of the same sex or not.
This is quite simple.
No, The IRS states that,
"For Federal tax purposes, the terms “spouse,” “husband and wife,” “husband,” and “wife” do not include individuals (whether of the opposite sex or the same sex) who have entered into a registered domestic partnership, civil union, or other similar formal relationship recognized under state law that is not denominated as a marriage under the laws of that state, and the term “marriage” does not include such formal relationships."
Roger Chartier - The Author