“Legally recognized “domestic arrangements” are just “marriage by another name.”
No, they weren’t. If that were the case, the LGBT legal activists would have pursued the repeal of DOMA, to satisfy federal recognition of civil partnerships, but they didn’t. They wanted to demand legally and socially the very redefintion of marriage, rejecting the legal substitute of civil partnerships.
What was and was not considered “equal” consideration, between civil partneraships and marriage varied between the states that had civil partnership laws. There are very few federal benefits or rights that extend to spouses outside of federal entitlement programs, which are about financial considerations. Extending them to civil parterships would not have changed the social and societal definition of marriage.
They simply are the same, or would be deemed so.