Why might the U.S. Supreme Court have said that gay marriage is a liberty that should be protected by due process of law?

Question

Why might the U.S. Supreme Court have said that gay marriage is a liberty that should be protected by due process of law?

Answer 1

The courts have recognized as fundamental rights, the right to travel and the right to marry. Even Charles Manson didn't forfeit his right to marry.

Life, liberty cannot be denied without due process of law. SCOTUS did rely on the 14th Amendment's Equal Protection Clause, but also on the 14th Amendment's Due Process Clause.