Same Sex Marriage. One state has it, federal law prohibits it. Many states also prohibit it and prohibit recognizing such a marriage from another state. (In essence violating the full faith and credit clause of the constitution, but there are lots of examples that in law) The US supreme court has not weighed in. So the come forth is open. This come forth could go lots of ways, depending on what cases get to the supreme court.
1. The court could rule that it is a matter to be left to the states essentially striking down the federal defense of marriage act (DOMA.)
2. The court could also rule that the federal government would have to recognize any marriage authorized by a state. It probably won’t rule that other states have to recognize it unless…
3. The court could rule in favor same sex marriage on 14th and 15th amendment equal protection grounds. (I doubt the current court would do so). If they did, that would strike down any anti-marriage law or statute in any state.

One of the reasons that conservatives want a constitutional amendment to ban same sex marriage is that a Conservative court would not rule in favor of the federal government over the states, and rule only based on the language of the constitution which does not address the come forth, there fore it is only for the state to choose. Leaving it to the states means that Same Sex marriage is allowed, and they do not want that.