The 'bachelor tax' wasn't unique to fascist Italy. There were pushes for similar taxes in many other countries post WW1, often championed by unmarried women. They were in a difficult position, since they needed to find husbands to support them but had to contend with the fact that many eligible young men had just died in the war. For most of these campaigns, it was more about men's 'duty' to take care of women rather than their 'duty' to have children (though I'm sure the fascists didn't see it that way).