That’s right. As the Tenth Amendment confirms, if it’s not expressly delegated to the feds by the Constitution, it’s an issue that belongs to the states and the people respectively.
Marriage also has a place in federal law, that is why the continental congress, and then the Congress was passing law related to marriage in 1780, 1894, 1798, and 1802, and so on, and is how President Obama has made such major rulings on gay marriage at the federal level.
That is why Rand Paul has become more open about his indifference to gay marriage and social issues, and abortion, as he runs to take Obama’s place.