Does the Catholic Church in the United States keep Thanksgiving and the Fourth of July liturgically? As if the violently anti-Catholic Pilgrim Fathers and the superciliously anti-Christian Founding Fathers were Saint? They were not.
"Today, America ceased to be a Christian country"? What, again? That does seem to happen an awful lot.
Today, in fact, the Supreme Court did exactly what it was set up to do, and specifically changed the legal definition from something that, incidentally, bore very little resemblance to Catholic Teaching, to something else that, incidentally, bears very little resemblance to Catholic Teaching.
None of the United States has ever had a law of marriage based in any way on that Teaching, just as no part of what is now the United Kingdom has had any such since the sixteenth century. The State has always done whatever it liked with civil marriage, just as it has always defined legal parenthood however it liked.
In doing those things, it can leave the Church alone, or it can drag us to the stake. We do not mind which. Or, at any rate, we should not mind which.
The only way to eradicate same-sex marriage is by the preaching of the Gospel, so that even if people could still it, they no longer would.
But put not your trust in princes. Or in Justices of the Supreme Court of the United States. Especially when they are only doing their job as the superciliously anti-Christian Founding Fathers defined it.