I think the point GP is trying to make is that sometimes state governments try to get involved in marriage and having a federal policy that preempts that can pr

I think the point GP is trying to make is that sometimes state governments try to get involved in marriage and having a federal policy that preempts that can prevent further meddling. 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.