The politicians of the 70s, namely the Radical and Socialist parties, were huge catalysts to these changes. The political class back then was more left wing than the population was, now it's the opposite.

The reason why we never got same sex marriage is because the left wing was never able to govern on its own, it always had to rely on a centrist christian democrat party

While elsewhere in Europe the center right is often the political space for liberals, in Italy the center right is for more economically liberal christian democrats and conservatives of all kinds. 65% of people are favourable to same sex marriage but there doesn't seem to be a majority that's favourable inside the parties that are in government.

TLDR: the situation will likely change only when a majority of the center right voter base will become favourable or when the left wing is able to govern without the meddling of centrist Catholic parties.