Union of Cyprus with Greece, just like union of Cyprus with EU, or union of Cyprus with anything else, should happen if this is what the Cypriots themselves democratically choose.
I believe that from 1950s (or even earlier) a referendum should have been allowed in Cyprus so the Cypriot people could vote and peacefully and democratically decide the destiny of their own island.
The resolution about De-colonization clearly states that "integration into an independent State" is a "legitimate option" for the people being decolonized to choose in a democratic way.
http://www.un.org/Depts/dpi/decolonizat ... ration.htm
Therefore union with Greece should have been an option in such a referendum.
Unfortunately the invaders of Cyprus didn't want to allow the Cypriot people to democratically decide the destiny of their own island, and they, using some small minority as an excuse, denied to the Cypriot people the right of self-determination.
If democracy had been allowed by the British and Turks (instead of insisting to impose their will by force) then nobody would suffer, there would be no conflicts and no wars and no 1974.
If the Cypriot people had democratically chosen union with Greece, then the Greek Cypriot majority and the Turkish Cypriot minority in Cyprus would continue to live on this island as they used to live under Ottoman or British rule, but this time as
free equal citizens of the state they
democratically choose to be part of, and not as subjects of a foreign empire impose on them by force.