No, that is not the reason behind the opposition. The reason comes from the Islamic laws in India that allow marriage of girls at the age of 15. To oppose this resolution, India would have to prohibit these marriages by law.
The contention given by you - seeing child marriages as mere engagements as they are not consummated till gauna, is a highly flawed one on two grounds:
1. Child marriages that you refer to, are unconstitutional and illegal and warrant immediate arrest. When they are already illegal in India, why would the nation abstain from voting in favor of a similar resolution on global level?
2. Such child marriages are not part of any Hindu tradition. Their main source is poverty, and complete authority of parents on their children. How do you suppose a girl can ever say no if the groom side wants to consummate the marriage at an early age?
Prohibition of child marriages does not mean adherence to Abrahmic values. It simply means giving greater rights to those children who never had any. In fact, Hindu Dharm, or Sanatan Dharm, too states the same, that no one shall be forced on to anything against the will, and that children are the souls that have no Dharm or caste and are free to live their life till they are of age.
This move by the government is merely to appease the Indian Muslim community, whose strange laws if tinkered with may result in the Political party losing its seat of power. In simple terms, vote bank politics.