Crimea is Not Montenegro, Kosovo etc. 

Ignoring the members of the He-Man Women-Hater's Club and their adoration of Putin's manliness (looking at you, Fox News and Stephen Seagal):

Crimea is Definitely Not Montenegro

In 2003, Serbia and Montenegro agreed to form a State Union to replace the increasingly moribund Federal Republic of Yugoslavia. That agreement (Article 60) specified that after three years, either republic could hold a referendum on independence. Montenegro, after careful planning and with widespread participation in the vote, voted for independence on May 21, 2006. Serbia was among the first states to recognize Montenegro's independence, acknowledging that the vote (closely monitored by the OSCE and other democratic states) was legitimate and in accordance with the 2002 agreement.

There is no agreement between Ukraine and Crimea, or Ukraine and Russia, that permits an independence referendum. The vote last month in Crimea has no more legal force than the online  poll in Venice that "also" declared "independence" (from Italy).

Crimea is Not Even Kosovo

Serbia, citing historical claims dating before 1389, annexed Serbia in 1912, after the First Balkan War against the Ottoman Empire.  The International Commission established by the Carnegie Endowment in 1914 said the heavy death toll among Albanian civilians in Kosovo was deliberate, resulting from Serb policy.  Oppression of the Kosovar Albanians continued, with thousands killed and tens of thousands forced out even before World War 2. Kosovo's autonomy, finally granted in 1974, was unilaterally revoked in 1989 by Slobodan Milosevic, the first abortive step towards a Greater Serbia and the Balkan Wars of 1990s.  Kosovo's 1990 declaration of independence was largely ignored, and Milosevic, believing himself to have a free hand in Kosovo after the 1995 Dayton Agreement, began to again crack down on Albanian separatism, leading to a flare up of armed conflict by 1997. In 1999, after (heavy-handed) efforts to impose a diplomatic solution failed, NATO intervened, with a race between NATO air strikes on Serbian armed resistance, and Serbian efforts to ethnically cleanse the region.

Kosovo is currently recognized as an independent state by 107 of 193 UN states, and as an autonomous self-governing region by Serbia.

Crimea was not best by government oppression directed by Ukrainian policy, and Russia rejected efforts by the international community to investigate, mediate and ameliorate such human right conflicts as may have existed.

Nor is Crimea Like Skane, East Prussia, etc.

Across the globe, there are many regions that used to be part of another state, much as Crimea was part of Russia before 1954.  E.g., Germany and East Prussia, Denmark and Skane (southern Sweden), Sweden and Finland, U.S. and New Mexico, U.S. and the Virgin Islands. In no way does international law recognize a right by the former owner state to reclaim its former territory, particularly if the state uses or threatens the use of force - as Russia did.