His government will not allow LGBTQ activists into schools, Orban said.
The right-wing leader was speaking on the day the new law entered into force. It bans schools from using materials seen as promoting homosexuality and gender reassignment, and says under-18s cannot be shown pornographic content.
It also proposes setting up a list of groups allowed to hold sex education sessions in schools.
European Union chief executive Ursula von der Leyen warned EU member Hungary on Wednesday it must repeal the legislation or face the full force of EU law.
But Orban said only Hungary had the right to decide on how children should be raised and educated.
The law, which critics say wrongly conflates pedophilia with LGBT+ issues, has prompted protests in Hungary. Rights groups have called on Orban's Fidesz party to withdraw the bill. The European Commission has opened an inquiry into it.
"The European Parliament and the European Commission want us to let LGBTQ activists and organizations into the kindergartens and schools. Hungary does not want that," Orban said on his official Facebook page.
The issue was one of national sovereignty, he said.
"Here Brussels bureaucrats have no business at all, no matter what they do we will not let LGBTQ activists among our children."
Orban, who has been in power since 2010 and faces a potentially tough election fight next year, has grown increasingly radical on social policy in a self-proclaimed fight to safeguard what he says are traditional Christian values from Western liberalism.
The opposition party Jobbik has also supported the bill in parliament.
On Thursday, the NGOs Amnesty International and Hatter society flew a huge heart-shaped rainbow colorhU balloon over Hungary's parliament building in protest against the law.
"Its aim is to erase LGBTQI people from the public sphere," David Vigh, director of Amnesty International Hungary, told reporters.He said they will not observe the new law nor change their educational programs.