Why doesn't the federal government define all legal benefits for civil unions rather than marriages?

I really don't know about this, but I thought I might share anyway. I understand that U.S. federal law has 1000 provisions that apply to married couples. The country has a strong history of separating church and state. Why does the federal government not change to make all these provisions apply not to "marriages" but to "civil unions". While the states could determine what they wanted to call a civil union, the federal government could define standards that state civil unions would have to meet to qualify people in their civil unions for federal benefits.

There are literally thousands of religions in the United States. Some sanctify relationships that the government finds contrary to public policy, such as polygamy. None of our governments are going to sanctify (in the sense of make holy) any marriage. We of course have to outlaw some cults, such as the Branch Davidians of David Koresh, the Jonestown sect of Jim Jones, or Charles Manson's family. Why should we let all the leaders of all these religions and sects determine who qualifies for government benefits?