On gay marriage, I believe that Gov. Rick Perry just signed a Defense of Marriage act, but yes, if the people of Texas were somehow insane enough to want that, sure. That's why social conservatives like me want to amend the Constitution with a Federal Marriage Act to stop that from happening in every state (I'm not worried about it being passed into law by a legislature; I'm worried about the kind of judicial activism we saw today).
Regarding marijuana, it gets a little more complicated. Again, I'm arguing from the 'original conception' of the Constitution that libertarians love to 'defend'. Under that regime, with a very narrowly interpreted ICC (interstate commerce clause) then yes. Today, no way. One state legalizing marijuana would never pass muster. You might want to note here that banning alcohol took a Constitutional Amendment (18th), but ever since roughly 1937 that kind of narrow interpretation of the ICC has kind of gone by the boards.