How quickly we forget that once we let the rabid dog run loose, it will sooner or later turn and bite us.
Defining marriage for "federal purposes" was a concept introduced by Republicans and passed by a majority Republican Congress under the name of the (Federal) Defense of Marriage Act.
I got kicked out of FR for pointing out that once we gave the Federal government the power to define marriage for Federal purposes, it would eventually define it in a way that we would oppose.
... and here we are.
P.S. There isn't now, any more than there was then, a "Federal purpose" for defining marriage. However marriage is defined by the individual State is the definition that the Federal government must work under.