Said this on the other thread too, but I'm generally mistrustful of any discussion on this subject that focuses only on marriage giving people rights, and doesn't mention that it also takes them away. The status quo means unmarried cohabitants also have rights that married ones don't, generally about their assets, and those rights are clearly very important to some of them.
However we do it, we're going to end up with some people who are disadvantaged. I'm not persuaded that creating a different set of problems to the ones that exist now is necessarily going to be an improvement.