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.
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.