Early returns indicate voters in Slovenia decided against allowing same-sex couples to marry and adopt children, the Associated Press reported. The referendum marked a major setback for gay-rights advocates in Eastern Europe.

Slovenia's Parliament passed a law in March that allowed same-sex couples to marry and adopt children, but a group called Children Are at Stake appealed to the country's top court calling for a referendum. Under Slovenia's Constitution, opponents needed to convince a majority to decide "No," totaling 20 percent of all eligible voters. AP reported authorities said 63.5 percent voted against a bill that defined marriage as a union of two adults while 36.5 percent voted for it. Those were not the final results, but AP reported the ultimate tally was unlikely to change significantly.

Preliminary referendum results show that Slovenians rejected a same-sex marriage law by a large margin: https://t.co/Fmx8v6q9F1 — The Associated Press (@AP) December 20, 2015

The referendum represented a significant decision in Eastern Europe, which has separated itself from Western Europe, where activists have been increasingly successful in pushing for same-sex marriage rights. Most recently, more than 62 percent of those who cast ballots in Ireland last May voted in favor of amending the country's constitution to allow gay and lesbian couples to marry. A number of other European nations such as the U.K., Spain and France have legally recognized same-sex marriages, but it remains a hot-button issue.

Gay-rights advocates lauded Slovenia in March when its legislature became the first in Eastern Europe to allow same-sex marriage. "We commend the elected representatives of Slovenia for passing such historic legislation ensuring the nation's [lesbian, gay, bisexual and transgender] citizens receive the rights they deserve, and we congratulate the LGBT activists and advocates who helped make this momentous day possible,” Ty Cobb, director of Human Rights Campaign Global, said at the time.

Slovenia had frequently been at the forefront in the region when it came to enabling gay rights, which may make the lopsided referendum results Sunday a bit surprising. "For a long time, Slovenia was first in the region in these issues," Roman Kuha, a commentator and sociologist, told the Financial Times before the results were released. "We established our first gay-rights organization in 1984, long before our neighbors."

With the reversal of the law, however, the country considered one of the more liberal in the region has joined a number of other nations there that have moved to forestall gay and lesbian marriages. Croatia and Slovakia recently amended their constitutions to prevent same-sex marriage, joining Bulgaria, Hungary, Latvia, Lithuania and Poland as countries that have restrictions defining marriage as a union between a man and a woman, the Financial Times reported.

In a referendum in 2012, about 55 percent of voters in Slovenia voted against giving more rights to same-sex couples, including the right to adopt the children of their partners, Reuters reported. The small nation of roughly 2 million people has been relatively accepting of gay couples, who have been allowed to register their relationships formally since 2006.