Arguable, at least not without some extra legal wrangling for things like power of attorney, etc. And the “secular” alternatives such as civil unions didn’t provide the federal benefits that are granted to married couples, because civil unions were state institutions. The benefits granted to civil unions depend largely on the state, ranging from non-existent (no recognition of civil unions) to being on par with the benefits of a full marriage. Suffice to say, much like the concealed carry permit issue, many folks aren’t big fans of suddenly losing legal protections when you cross state lines. And even then, you were still missing federal benefits.
So no, there were legal benefits that were not available outside of marriage.
Frankly I’m of the opinion that SSM wouldn’t have become a thing had federal civil unions been recognised and given the same benefits as marriage.