If you get government out of the marriage business, marriage stops being a legal reality. You therefore get rid of the legal status of people who are married. You take away benefits and legal status from straight people who are already married.

No amount of sugar makes "oh, by the way, we don't recognise your marriage anymore" taste any better.

> If you get government out of the marriage business, marriage stops being a legal reality.

Not at all. There are lots of legal realities now that have nothing to do with the current marriage-specific laws, and likewise marriage could exist as a legal reality without the current marriage-specific laws.

Then you have government blessing certain types of unions for specific purposes. At a certain point, the government has to decide whether I get an inheritance tax benefit for my partner's estate and they will have to decide whether to grant that to long-term same-sex couples or restrict it to just opposite-sex partners.

Which is basically the same thing as having a set of marriage laws without the ceremony and the cake. Pointless refactoring for the sake of it.

> Then you have government blessing certain types of unions for specific purposes.

Not for most things. You just make a contract with someone, and the government can (if necessary, through the courts) enforce that contract. That works fine for hospital visitation and medical decision making (the government can step in if a hospital refuses to respect a contract) and wills. For things like tax exceptions, well, that's all the government's doing anyway. How about just give everyone the tax benefit if it's really so complicated to figure out.