Last year, the Slovenian parliament passed the law allowing homosexuals to adopt the biological children of their partners. But the Civil Initiative for Family and Children's Rights challenged the law and put it on the referendum ballot after gathering 40,000 signatures on a petition.
"The law must be rejected since it does not respect the basic rights of children," said Ales Primc, head of the Civil Initiative group.
The law did not allow a same-sex couple to adopt children from a third party.
Preliminary results released by the state electoral committee showed 56.5 percent of voters were against the law, while 43.5 percent supported it. Ninety percent of votes had been counted, with only about 26 percent of eligible voters casting their ballots.
mz/tj (AP, Reuters, AFP)