Good grief! Defining marriage is an act of regulating marriage which the states have never delegated to Congress, via the Constitution, the specific power to do.
When Alito says the "definition of marriage" under that kind of context it's clear that the definition is expository, falls clearly under the judicial discretion afforded to terms in common use, and is not a regulation. I know you've got an untenable position to defend, but please stop being outrageously silly or I'll have to stop discussing this with you.