The pontiff made the comments in the Vatican courts, which mainly deals with marriage annulments, ahead of the vote this Thursday.
Italy is the only country in Western Europe where both civil partnership and gay marriage do not exist.
Some believe civil partnership would be another step towards gay marriage in Italy.
The Italian Prime Minister Matteo Renzi supports civil unions, but other traditional colleagues in his centre-left Democratic Party do not, as well as politicians from other parties in Italy.
Pope Francis told Vatican Radio: "The Church... can show the unfailing merciful love of God to families - especially those wounded by sin and the trials of life - and, at the same time, proclaim the essential truth of marriage according to God's design.
"When the Church, through your service, sets about to declare the truth about marriage in a concrete case, for the good of the faithful, at the same time you must always remember that those who, by choice or unhappy circumstances of life, are living in an objective state of error, continue to be the object of the merciful love of Christ and thus the Church herself.
"Therefore, the Church, with a renewed sense of responsibility, continues to propose marriage in its essentials - offspring, good of the couple, unity, indissolubility, sacramentality - not as ideal only for a few... but as a reality that, in the grace of Christ, can be experienced by all the baptized faithful."
