The opposition far-right People’s Party Our Slovakia also voted against it.
Opponents of the convention in Slovakia, which is predominantly Roman Catholic, charge that some provisions would violate the Slovak Constitution — for example because the constitution defines marriage as a union of a man and a woman.
In a separate vote just days before Saturday’s parliamentary elections, the two coalition parties also voted with some others to approve legislation that gives retirees an extra monthly pension — 13 instead of 12.
The center-right opposition parties that stand a good chance to form a government condemned the move as populistic.
It wasn’t immediately clear if Caputova, the president, was ready to approve it.