According to ISN Security Watch:
Officially speaking, the government failed after Prodi's allies fell two votes short of the majority needed to gain approval on a vote backing Italy's peacekeeping role within the NATO-led mission in Afghanistan. But while the debate over Italy's 1,900 Afghan-based troops may have been the final act in Prodi's nine-month tenure, the real culprit now appears to be Prodi's plans to legalize same-sex marriages in Italy..
In retrospect, it did not go well at all. Abandoned two days later by three proudly Catholic Senators, the Prodi coalition's modest majority in the Senate became a two-vote minority, and the Afghan security vote failed (the left-leaning daily newspaper La Repubblica noted that all three Senators had backed similar policies in the past).
After several long nights of negotiations, Prodi's backers and several fence-sitters all signed off on a 12-point plan for governance essentially identical to the plan he adopted before his resignation, save one conspicuous absence: the plan to legalize civil unions, including same sex unions - the one government plank strongly opposed by the Vatican - was nowhere to be found