Bishop Elio Sgreccia said this at the launch in Rome of Carlo Casini's book "Unioni di fatto, matrimonio, figli: tra ideologia e realtà" (De Facto Unions, Marriage, Children: Between Ideology and Facts), published in Italian by Società Editrice Fiorentina.
Casini is a member of the European Parliament representing Italy, and director of the Italian pro-life group Movimento per la Vita.
During the press conference held at Vatican Radio, Bishop Sgreccia said that he is not proposing that Church interfere in state affairs.
"International and national legislation defend the family as 'the natural and fundamental group unit of society,'" he said, pointing to the Universal Declaration of Human Rights which says that "the family is the natural and fundamental group unit of society and is entitled to protection by society and the state."
Bishop Sgreccia explained that the types of families proposed by the de facto union laws do not "belong in the concept of marriage as stated by the international society and the Italian republic."
He warned: "If family stability is weakened, the fundamental group unit of society is also weakened."
Regarding homosexual unions, the president of the Pontifical Academy for Life said that this goes "against a law of nature."
Bishop Sgreccia underlined that "in the corporality of man and woman there is a written natural and structural complementarity pertaining to emotional life, sexual life and the procreation of children."
According to the bishop: "Natural law expresses the good sense" of recognizing "the union between man and woman."
Bishop Sgreccia commented on the importance of the family for the economy and social progress of nations, citing the work of Gary Becker, winner of the Nobel Memorial Prize in Economic Sciences in 1992.
Becker's thesis, explained in the book "Human Capital," states that to have a healthy economy, there must be a stable and healthy family, not broken up or hurt by conflicts, divisions or instability.
Bishop Sgreccia stated: "If we wish to see an economy that has as its main unit family health, then we must take into account that the de facto families, constitutionally precarious, are the source, as well as divorce and separations, of social and economic instability."
According to the bishop, instead of inviting people to form de facto couples, "the state would do well in helping youth by teaching them the formation of a true family, with a civic or religious marriage, but stable, and with an eased access to house and work."
He continued: "This is where the money should be spent, and not creating marital precariousness."
Bishop Sgreccia said that by allowing de facto unions, the state is not helping youth, but rather "placing them in a situation that does not give security, that does not give stability or serenity."
Regarding homosexual couples, the bishop said that "gay pride certainly does not help them in overcoming their suffering, which instead must be faced with human understanding, with medical and psychological sciences, and with promotional attitudes of all the good qualities that exist in these persons."
Bishop Sgreccia added that "Catholics will do everything to shed light on the debate, and will do everything possible so that these proposals will not pass."
He added that "everything based on untruths is destined to damage someone, if not many people."