Supporters say fidelity should not be thought of only in sexual terms.
"The obligation to fidelity should be understood not only as sexual fidelity but above all respect and trust in each other, which is an important value which should not be up to the state to impose by law," the proposal on the law says.
They have also referred to a previous ruling from Italy's highest court, which declared that judges could not legally place the blame for a marriage separation "on the mere failure to observe the duty of fidelity".
Instead, the other party has to prove their spouse's infidelity led to the irreconcilable breakdown of the marriage.
In May, Italy became the last western European country to allow civil unions between homosexuals after fierce resistance from the Catholic Church and conservatives.
References to fidelity and faithfulness were removed from the bill despite an outcry from the gay community seeking parity with homosexual relationships.
Fidelity has always been a topical issue in a country where the affairs of 18th-century adventurer Giacomo Casanova made his surname synonymous with betrayal.
According to a 2014 poll, 55 per cent of men and one in three women in Italy said they had cheated on their partners.