Just over half of the members of the Council of Europe have ratified the human rights watchdog's 2011 Istanbul Convention, which is the world's first binding instrument to prevent and combat violence against women, from marital rape to female genital mutilation.
While there is no explicit mention of gay marriage in the treaty, many Slovaks view its wording as a threat to the traditional family structure.
"The convention talks about stereotypes and gender equality in the sense of eliminating the so-called traditional roles of men and women in the family. It raises doubts," Fico told reporters.
"Unless there is full compliance of the provisions of the convention with the definition of marriage as a bond between a man and a woman, I will never agree to ratify."
Slovakia's parliament amended its Constitution in 2014 to define marriage as a union between man and woman, which stirred protest among rights groups at the time.
Though against ratifying the convention, Fico said he was committed to incorporate some of its elements into Slovakia's domestic law.
"We will adopt all the necessary legal regulations to ensure that our legislation is at the European level when it comes to the protection of women against violence," he said.
Seventeen EU members have ratified the Istanbul Convention, along with non-members Albania, Andorra, Bosnia and Herzegovina, Georgia, Monaco, Montenegro, Norway, San Marino, Serbia, Switzerland and Turkey.
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