I will come to that in a moment.
Perhaps the parts of society that do not view civil partnerships as being exactly identical to marriage do so because a large proportion of society views marriage as being about the union of a man and a woman for the creation and care of children, and not simply about the love and commitment of the happy couple, as important as that is. On the other hand, civil partnerships are a celebration and recognition of the love and commitment that two people of the same gender have for each other.
The state has sought to treat marriage in a special way in recognition of its intrinsically child-centred nature. That is the only reason why the state has previously had any interest in marriage at all. If marriage were simply about love and commitment, we would first have to define love as being sexual love, because otherwise non-sexual relationships that are based on love and commitment would also have to be treated as marriage on the basis of the definition of equality. If the definition of marriage is simply love and commitment, why is the state interested at all? What business is it of the state’s to register and record such unions? It is because marriage is about so much more that the state has historically wanted to be involved.