Explain That To Me

[Monsignor Juan Antonio Martínez Camino, the secretary general of the Spanish Bishops Conference,] called the 2005 [Spanish] law legalizing gay marriage and adoption "very strange and very irrational and very unjust."
For Vatican, Spain is a key front in church-state battle.

Okay. Strange, I get. There aren't that many countries in the world that have legalized homosexual marriage. Irrational, I get. If you believe in a deity that punishes people for sins (and societies for legalizing sins), such a law would be irrational from the point of view a a devout believer.

Unjust, I don't get. I don't see the injustice that's being done to anyone with allowing same-sex couples to marry. Of course, the International Herald-Tribune gives us only a snippet of the Monsignor's comments - he may well have gone on to elaborate his message in a way that would have made his logic clear. But in a sound-bite culture, this is, I suspect, part of the problem that the Church has with charges of bigotry - claiming, in effect, that anything that contravenes Church doctrine is necessarily unjust seems well into the territory of sanctimony and self-righteousness.