When abortion was legalized in Belgium in 1990, King Baudouin, a devout Catholic, temporarily abdicated rather than sign his name to the law. If he had been an "unconstitutional" monarch with real power, abortion would still be illegal in Belgium.
The current Belgian king did not follow pious Baudoin's good example in the most recent laws that were controversial as far as Catholic doctrine go, I understand. I think it was about euthanasia (on which Belgium has one of the most extreme positions), or same sex marriage.