Trying to think of analogy here but nothing concrete comes up. The messy thing about this is that the ethnic, cultural and linguistic makeup of Crimea is by majority, Russian. It's something of an foreign-induced rebellion that the majority willingly accept.
So, let's say Crimea overwhelmingly wishes to secede to Russia. Would it be undemocratic for the US to oppose such a move, whether it is induced/heavily influenced by a foreign nation? It get's murkier at that point. It's a rather well-planned move on Putins part and I believe it was pre-mediated months in advance. But that's just my opinion.