“Marriage regulation in the US is a state law matter. [..] Believe it or not, family/marriage law is one area the feds don’t want to get involved in, …”
— Comment by thedeti @ Sigma Frame “A Concise History of Marriage Regulations” (2023-06-12)

In 1996, federal lawmakers successfully passed the Defense of Marriage Act into law to ban same-sex marriage. A majority of states passed similar constitutional amendments regulating marriage by banning homosexual unions. Then the Supreme Court enshrined the federal Constitutional right to same-sex marriage:

In Obergefell v. Hodges (2015), the Court held that same-sex marriage was a fundamental right protected by both the Due Process Clause and the Equal Protection Clause. The ruling required all states to perform and recognize the marriages of same-sex couples, leaving Section 2 of DOMA as superseded and unenforceable.

While DOMA would eventually be repealed in the “Respect for Marriage Act” in 2022, the Constitutional right to same-sex marriage is now federal law, overriding all existing State constitutions to the contrary. Marriage regulation is thus very much a key part of federal law and is fundamentally not solely a state matter.

When it wants to, the federal government can and will regulate marriage, whether directly (e.g. DOMA; same-sex marriage) or indirectly (e.g. tax law). According to the American Bar Association, this includes a wide variety of  federal family laws.

What the federal government chooses to regulate only tells you what its priorities are, not its functional ability to do so, which it has aptly demonstrated and keeps expanding over time. Everyone knows that marriage regulation in the U.S. is a only matter of state law at the allowance of federal discretion. It is only a practical—not in principle—matter of state law.