My understanding of the government's position on heterosexual civil partnerships is that they're claiming that because of the extension of marriage to same sex couples, that there's no longer any need for civil partnerships at all. Everyone should just get married.
That seems a really faint basis to uphold a logic that was most commonly used to oppose legalising gay marriage: that it's possible to define the sex of participants in a legal union in such a way to exclude certain couples. Civil partnerships are currently closed to heterosexual couples on the basis that they are only between two people of the same sex, the opposite of which was asserted in opposition to the legalisation of gay marriage (ie. Marriage is between a man and woman only).
As such, this appears to be a manifest injustice, flying in the face of the precedent set by the legalisation of same sex marriage, that one cannot specify the sex of participants to specific legal unions without discriminating and therefore treating people unequally. The upholding of the ban on heterosexual civil partnerships operates on the same logic.