Rule of law is founded on the principle that the people, or their elected representatives, make decisions, and judges enforce those decisions. Fundamental rights have to be decided and agreed upon by the people (ideally with a supermajority) before being codified in a constitution, at which point they are to be enforced. If this principle is turned on its head, is opens the door for judges to declare anything they want to happen a "fundamental right" and become unelected lawmakers.
Now, maybe the Austrian constitution actually does mandate equal marriage rights for same-sex couples - in this case, kudos on the judges for doing their jobs. But given when it was written, I highly doubt that's the case.
Sciences Po Paris is really bad at teaching the three powers theory then. Juridicial power isn't subordonned to legislative power.
