The 1996 Defense of Marriage Act was a big accomplishment for Newt Gingrich's Republican controlled Congress.  It says that the federal government does not recognize same sex marriages, period.  You'll notice that Conservatives are big supporters of state's rights, up until states do something they don't like.  The author of that bill, Bob Barr, hilariously became the Libertarian Party presidential candidate in 2008.  

Only another law, passed by Congress, is going to change the situation. 

Same-sex couples must file federal tax returns as single even where gay marriage is legal.