Pure fascism, and it WILL NOT STAND in Greece. Still very religious country. They have blasphemy laws. If the government acquiesces to this, there’ll be an uprising.
Up till recently, despite all the financial troubles, the polls showed slight preference for staying in the EU in Greece. Remember, Greeks are by nature colonizers: they emigrate easily. The ability to get to London or Paris and work, and keep the Greek passport is of immense importance to many Greeks.
Also remember that this is not a mandate to have a gay “marriage”, but rather a ukase that IF Greece has a civil union legislation at all: that is if it gives a legal status to cohabitating unmarried couples, THEN that legislation should recognize civil unions of gays. In a religious country, a cohabitating couple is not following the divine law anyway.