I don’t think the government has thought far enough outside the box on the issue of marriage.
We need to take the sex and the religion out of partnership contracts.
In a secular society it seems ridiculous that some religions still retain the right to authorise legally binding contracts. After all, religious christenings or namings and funerals no longer count as legal registrations of births or deaths.
Marriage constitutes a legal partnership contract for the purposes of child custody, inheritance, and next of kin rights, thus it seems ridiculous to allow the functionaries of some religious and secular groups but not others to authorise such contracts. I doubt that my Archdruid or my Pact Bishop would get a license to dispense legally binding contracts of handfasting or marriage, so why should any other religious or humanist organisation?
If people want to seal a partnership with a contract then it should remain a purely legal matter. They can always have any sort of religious or secular celebration that they can persuade anyone to participate in, before or afterwards. If some religions want to decline to have some types of celebration then that’s their choice, and their loss of revenue.
Plus the terms of contract require re-examination and greater flexibility. My widowed mother cannot marry her partner because both have children and property, and if either dies their property lies open to claim from the other or the other’s adult children. Pre-nuptial contracts have no force in law in the UK.
My mother also lost her inheritance when her widowed father re-married; the courts over-ruled his will assigning his property to his own children on the death of his new wife.
Also there was the celebrated case of two sisters who had been lifelong companions and when one died the other had to sell their house because she couldn’t inherit the other half of the house without paying death duties. If they had been two unrelated women they could have had a legal partnership and the right to inherit each others property.