Comparing the Netherlands to the entirety of the USA is just weird. What are the approval ratings in New York state for instance? That is closer in population to the Netherlands (and potentially more rural & thus more conservative but still likely closer).
On most day-to-day reality issues (for same sex couples or anyone else) local norms are probably much more important and I can assure you there are lots of parts the United States that have 90% approval rates for same sex marriage.
So if we are discounting the legal perspective (which just as a personal opinion, I wish the US federal law was more progressive), cherry-picking public opinion to match the more progressive parts of the sample seems like the US comes out about the same.