Initiative representative Zeljka Mrakic congratulated the judges, saying they had resisted political and other pressure and confirmed that Croatian voters, when abiding by the law and the Constitution, had the right to decide how to organise their country.
Speaking to Hina, she said the Court's decision confirmed that everything related to the initiative was in line with the Constitution and that "it doesn't violate human rights and that there is no discrimination here."
The initiative collected 740,000 signatures for a referendum whereby the Constitution would define marriage as a heterosexual union.
The Kontra lesbian association said the Constitutional Court had failed to protect the human rights of a minority and that state institutions were hiding behind procedure, refusing to take a position on the referendum's question as a human rights issue.
Kontra representative Sanja Juras said LGBT persons in Croatia were already disenfranchised and treated as second class citizens and that the Constitutional Court should be a guardian of equality and human rights.
She said the referendum represented the harassment of a minority and that it was aimed at permanently stopping progress in the protection of its human rights.
Kontra called on citizens who cared about equality and human rights to go to the referendum and vote against, and do what state institutions had failed to do - defend human rights.
The association called on citizens to take part in the "I vote against!" march on November 30, a day before the referendum.
Zeljko Peratovic of the Centre for Civil Courage said the Constitutional Court had acted illegally, just like parliament and the Administration Ministry, with regard to the referendum.
"This procedure opens the door to a broad clerical campaign that will result in other demands for the restriction and prevention of the secular state," he said.
The GONG association said the Constitutional Court's decision was expected but disappointing.
However, its executive director Dragan Zelic said it was good that the Court had stated that marriage and family were not synonyms, that the referendum's decision must not impact the development of the rights of common marriage and same-sex unions, and that everyone was entitled to the protection of their private and family lives and human dignity.