Simple justice and compassion.
As a happily married heterosexual I am disconcerted that my homosexual friends' partnerships cannot be described in law as a 'marriage'. (No doubt because there are some minor differences between the two forms of legal status.)
I would like to see civil partnerships put on exactly the same basis as conventional marriage from a legal point of view.
Such a change need not affect the rights of the religious to continue to confine the term 'marriage' to whatever they want, but that is a matter for them and need not concern the rest of us.