Child abuse is a heinous and personally damaging crime; it is therefore incumbent on the Church to treat such matters with the utmost seriousness.
If the rights of civil partners are met differently in law to those of married couples, there is no discrimination in law, and if civil partnerships are seen as somehow 'second class' that is a social attitude which will change and cannot, in any case, be turned around by redefining the law of marriage.
Would we be a better society if we made marriage simply a private contract between two individuals, with no wider implications of kinship and family? I do not believe that we would.