The real shift has happened in the last 20 years or so, pretty fast die-off so I be thinking it isn't young people but ALL people and the majority will not see anything being forced down any throat but a long awaited equality that was too long in coming. Just like interracial marriage.
But as usual you ignore the crux, some states will never permit SSM and that isn't how the UNITED STATES works.
But it is a bit amusing to see 'conservatives' bemoan the government creating 'artificial legal rights' to mold society when it is a benchmark talking point of the 'conservatives' since I can remember. Seems to me that now with those 'artificial legal rights' being extended to all who wish to marry, not just the approved by the right wing religious crowd, some 'conservatives' are bored with the whole thing and want all legal rights revoked.