This is an act of moral cowardace (1) and compromise (2) . Cowardice by any king is unsavory, and unworthy.
Theodore Harvey has written an opinion piece that sees the King in a far more better light that I am afraid I can. Theodore Harvey does not "feel it fair to blame the King". Why is it that we have always to blame someone? In this case, it is because by his signature he has condemned thousands of babies to be killed in his name. Truthfully the Spanish Government could after voting removed the requirement for his signature altogether such as in Luxembourg where the legislature has removed the Grand Duke Henri's right of veto. This is, I am afraid, one of those times where there will be no ambiguity at the judgement.
It seems that Theodore Harvey is willing to congratulate King Juan Carlos for his "prudence". I fail to see the the prudence in spending an eternity in hell for bowing to the secular god.
(1)...Moral cowardice is fear of upholding the good because it is good, and fear of opposing the evil because it is evil... Ayn Rand
(2)...There can be no compromise on basic principles. There can be no compromise on moral issues. There can be no compromise on matters of knowledge, of truth, of rational conviction... Ayn Rand