thank you for your question:
This is what I found online, I hope that helps you and your fiance:
"Registration of marriage
According to the Italian Civil Code, foreign citizens who intend to contract a marriage in Italy must present a certification of the absence of impediments to contracting the marriage (nulla osta), issued by their embassy.3 Refugees can substitute the nulla osta with a UNHCR certification. This practice was established following a formal note sent on 9 April 1974 by the Ministry of Justice to the Ministry of Foreign Affairs, copying UNHCR.
In order to obtain authorisation for the marriage, refugees must produce:
A declaration (affidavit), signed before the Civil Court or before a notary and certified by two witnesses;
The decision granting them refugee status;
A valid residence permit; and
A valid document of the future spouse.
The law does not provide a solution for beneficiaries of subsidiary protection who cannot request the nulla osta from their embassy with a view to registering a marriage. In this case, they can follow the procedure set out in the Article 98 of the Italian Civil Code, which entails a request for the marriage authorisation to the municipality and, after the refusal of the request for want of nulla osta, an appeal to the Civil Court, asking the Court to ascertain that there are no impediments to the marriage.
With a decree issued on February 2012, the Civil Court of Bari has authorised the marriage between a subsidiary protection holder and an asylum seeker even in the absence of authoriation from their country of origin. The Court observed that in relation to the certification needed for contracting a marriage, “regugees and subsidiary protection beneficiaries appear to have similar positions, but unjustifiably treated in a non-homogeneous way…”4
In your Situation you will have to obtain certain documents as well as mentioned above:
"According to the Italian Civil Code, foreign citizens who intend to contract a marriage in Italy must present a certification of the absence of impediments to contracting the marriage (nulla osta), issued by their embassy.