This would provide equality to a disadvantaged group and would ensure that the UK does n't get left behind and look hypocritical when it speaks about promoting equality.
Civil partnerships should only be seen as a stepping stone to full equality. Gay men and lesbians are being dsicriminated against and put at great disadvantage because their marriages are given no legal recognition. People who have fought hard for the right to marry in any of the progressive countries that do now recognise same sex marriage do not get legal recognition in the UK and it is unsulting to expect them to accept civil partnership as a form of recognition when they have fought to upgrade this to marriage. Also people married in churches such as the MCC or Quakers want legal recognition of their marriages and will not accept second rate civil partnership so are afforded no protection or rights.
A second rate separate system is an insult. The litmus test for eqaulities situations is to compare with other equalities groups. If mixed-race marriages were downgraded to civil partnerships, or the marriages of infertile heterosexuals there would be outcry. This should be no different.