People misunderstand the legalization of gay marriages at the stat level and not at the federal level. An article was written at KPSP Local 2 news website does a great job explaining the differences.
As of right now same sex marriage marriage is not recognized under Federal law. Here is a look at some important differences that will affect gay couples:
Federal Income Tax laws make a sharp distinction between those involved in a traditional marriage and those in a same sex marriage. While married gay couples must file their state tax returns as married filing jointly or separately, they must file their federal income taxes as single or head of household. This can lead to more Federal taxes being paid by gay couples.
Immigration laws treat same sex couples differently as well. If an American citizen marries a foreign citizen of the same sex, the foreign citizen will not gain residency rights from the marriage, as would a spouse from a traditional marriage.
While surviving male/female spouses are entitled to a dead spouse's Social Security benefits, this is not the case with same sex couples. A deceased gay spouse's benefits could go to another relative, or could escheat to the state instead.
While a surviving straight spouse can roll over a deceased spouses individual retirement account into his or her own IRA, thereby preserving the tax benefits, a same sex spouse or domestic partner cannot. He or she must begin taking fully taxable distributions from the inherited IRA
One more difference between domestic partnerships and gay marriage. Domestic partners must live together when their partnership is established. This is not a requirement for same-sex married couples.