BUCHAREST, Romania — A referendum in Romania on whether to narrow the constitutional definition of a family to a man and a woman failed spectacularly over the weekend, with just 20.4 percent of the voting population taking part.

Romania is one of six European Union countries that does not allow same-sex marriage or civil unions. But its Constitution uses gender-neutral language to define “family,” and conservative groups, fearing that could lead to legal recognition for same-sex relationships, sought the referendum.

With opposition groups boycotting the vote, the new constitutional language was expected to win the support of a majority voters. But turnout was so low that the referendum results appeared invalid, leaving the existing definition of family in place.

The turnout was far below 30 percent, the threshold for a referendum to be binding, despite a decision by the government, which had largely backed the referendum, to allow two days of voting rather than one. Final results are expected to be released on Monday.