So that my marriage is recognized across state lines, for a start.
So that my marriage is recognized across state lines, for a start.
This cuts both ways—with preemption, you can provide baseline rights or guarantees to citizens. The trade-off is that you have federal legislation in the mix and you then need to deal with laws that are slower/harder to change; a big issue if the law was badly written or needs to be changed in a timely manner.
This argues for federal legislation that defines marriage simply as a compact between two consenting adults with some basic legal record keeping.
The /impact/ of that marriage can be both federal and state (e.g., federal vs. state tax laws).
Individual state laws defining marriage could mean your next of kin could change if you die in the wrong state. That way lies dragons.