Last September the UN Human Rights Council adopted a resolution "Strengthening efforts to prevent and eliminate child, early and forced marriage". India now offers justification for its refusal to co-sponsor this resolution: "The concept of 'early marriage' is nowhere defined and it is debatable whether early marriages can be 'eliminated'... Consequently, there was need for greater legal clarity for such a term and its usage".
The real reason is nevertheless crystal clear - India's poor record on preventing child marriage, which according to US State Department's Country Report on Human Rights Practices in 2012 includes the following:
"Child abuse and child marriage were problems...The law does not characterize a marriage between a girl below age 18 and a boy below age 21 as 'illegal'... UNICEF's State of the World's Children 2012 report stated that 43 percent of women were married before age 18. On August 8, the MWCD [Ministry of Women and Child Development] informed parliament that there were 113 incidents of child marriage reported in 2011; however, it noted that sample surveys on health indicators suggested the 'prevalence of child marriages which are not registered.'"