The reasons appears to be cultural, political and religious.
Some states and countries have legalized same-sex marriage. Some states and countries have banned same-sex marriage by statute and some by amending their constitutions.
The states that have legalized same-sex marriage did so because either the legislature or judiciary came to the conclusion that the state had no legitimate or compelling reason to deny marriage rights to same-sex couples.
The states that have legalized same-sex marriage did so presumably because either the legislature or the judiciary decided there is no longer had any legitimate interest served by limiting marriage to opposite-sex couples only.
The states and countries that have legalized same-sex marriage have not reported any negative effects.
There is no legal reason for it to be illegal. That is why in states where someone has sued to get the law changed is has been.
In some states with common law provisions, this could be a possibility. However, to be recognized for tax purposes a marriage must be legalized through a marriage license.
The countries that have legalized same-sex marriage did so because the legislature came to the conclusion that the government had no legitimate or compelling reason to deny marriage rights to same-sex couples.
States that have not legalized same-sex marriage are affected by the legalization of same-sex marriage in nearby states as follows:Non-marriage states loose tourism dollars to marriage states.Politicians in non-marriage states are subjected to increased pressure to address the issue (think of Rhode Island being the only state in New England that has not legalized same-sex marriage).Some gays and lesbians choose to move from non-marriage states to marriage states. These are usually the most wealthy, best educated young people who have the most to contribute to a state's economy over the course of their lives. Poor, uneducated older folk stay put.Businesses complain that a patchwork of domestic partner and civil union laws make managing Human Resources more difficult and legalizing same-sex marriage would ease this burden.Businesses claim that the legalization of same-sex marriage makes it easier for them to attract and retain highly qualified workers. One can assume that banning marriage has the opposite effect.
In states that have legalized same-sex marriage, there is no legal difference between the two. In states that have banned same-sex marriage, the difference is that heterosexual couples are permitted their civil rights.
Same-sex marriage has been legalized in at least some countries (or states) in North America, South America, Europe and Africa. One country in Oceania (New Zealand) is scheduled to begin marrying same-sex couples on August 19, 2013. Several territories in Antarctica permit same-sex marriage, although marriages almost never happen there. Asia is the only continent where no country has legalized same-sex marriage.
Washington, DC, which is the only city in the United States to have legalized same-sex marriage. The District of Columbia is the only city in the country that is part of any state. In the United States, marriage laws are controlled by the states, not cities. No city has the authority to pass laws concerning marriage. This is why some municipalities have passed local laws concerning domestic partnerships.
These US states have legalized same-sex marriage:Massachusetts - May 17, 2004Connecticut - October 10, 2008Iowa - April 27, 2009Vermont - September 1, 2009New Hampshire - January 1, 2010Washington D.C. - March 3, 2010New York - July 24, 2011Washington (state) - December 6, 2012Maine - December 29, 2012Maryland - January 1, 2013[Same-sex marriage was previously legal in California, but is no longer permitted pending a decision by the United States Supreme Court.]These US states have legalized civil unions:New Jersey (February 19, 2007);Illinois (June 1, 2011);Rhode Island (July 1, 2011);Delaware (January 1, 2012); and,Hawaii (January 1, 2012).[Some states that legalized civil unions later legalized same-sex marriage and abandoned civil unions.]