I suspect that we will have civil unions -- termed marriages -- between same sex couples in many states in the not-to-distant future. In practice, this will spread to all states.
The issue is emphatically NOT addressed in the U.S. Constitution, therefore the 10th ammendment notes that it is up to each state to decide such things for itself in accordance with its laws, so long as those laws/decisions do not contravene the rights of another U.S. citizen. Since extending "marriage" to include your same-sex couple does not impinge on my rights directly (doesn't make my marriage any less meaningful/beneficial), states may decide to make such unions legal.
Where it will get interesting is the "full faith and credence" clause to the U.S. Constitution. I suspect that -- absent some compelling data that same-sex marriage somehow debases and harms traditional marrriage (thus constituing a impingement of another's rights) -- ALL states will be required to treat such marriages as legal unions with all rights, duties, and privileges thereunto apertaining EVEN if the state in question opposes such unions and does not allow them to be performed in that state.
I suspect that the court would have to rule thus even now based upon Massachusett's decision alone.
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