The answer to that question depends upon where you live. In many places in the world it has already been legalized, although in most places it has not. The rate at which new places legalize it seems to be accelerating, with 2013 being the top year so far.
The truth is that same-sex marriage is probably inevitable in all of North America, most of South America, and most of Europe and Oceania. There is almost no progress being made in Asia and Africa.
The Netherlands was the first country in the world to legalize same-sex marriage in 2001. Same-sex marriage was first legalized in the USA in the state of Massachusetts on May 17, 2004.
Yes. A lesbian can marry a man anywhere. A lesbian can marry a woman in any of the states and countries that have legalized same-sex marriage.
If by "illegal" you mean "against the law," then none. There is no state in the US where same-sex marriage is considered a crime. In fact, in most US states, same-sex marriage is legally recognized. (See related questions below: "What US states have legalized same-sex marriage" and "What US states have banned same-sex marriage.")
Yes, in places where they have been legalized (New Jersey, Hawaii, Illinois, Delaware, Rhode Island and New Zealand). Also, they are usually recognized as legal marriages in states where same-sex marriage has been legalized. Some states recognize out-of-state civil unions as domestic partnerships.
The minimum age for marriage in all US states that have legalized same-sex marriage is 18. (Younger persons can marry with parental consent, judicial consent, or both.) However, a 16-year-old can marry in Québec, Canada. Same-sex marriage is also legal in Canada.
No countries in the Balkans have legalized same-sex marriage.
Same-sex marriage has not been legalized in Laos.
No country in Central America has legalized same-sex marriage.
No former Soviet republics have legalized same-sex marriage.
No predominantly Muslim countries have legalized same-sex marriage.
Same-sex marriage was legalized in North Carolina on October 9, 2014.
Same-sex marriage was legalized in British Columbia on July 8, 2003.
Same-sex marriage was legalized in the Canadian province of Saskatchewan on November 5, 2004.
As of December 20, 2013, about 33% of US states have legalized same-sex marriage.
Same-sex marriage was legalized in the Canadian province of Nova Scotia on September 24, 2004.
Same-sex marriage was legalized in the Canadian province of Newfoundland and Labrador on December 21, 2004.
Same-sex marriage was legalized in the Canadian province of New Brunswick on June 23, 2005.