Simply put: two of the five justices who voted in favor of Obergefell (the case that legalized same-sex marriage) are no longer on the Court. They have been replaced with justices who would be more likely to vote the other way.

The realignment of the Court, plus this Court's willingness to overturn settled law, together signal that a new law prohibiting same-sex marriage might be allowed to stand if brought before today's Court, returning the US to a country where same-sex marriage is not universally legal.

So I would regard our current regime around same-sex marriage, civil rights, etc. to be subject to not being directly challenged at the Supreme Court.