I would argue that science is the best method we have of keeping our beliefs honest by constant examination of the data -- and that data as concerns the cohabitation of couples, or triples for that matter does not in any way suggest that the Holy Father's predictions are based on what happens in this world when such things are not prohibited by law or even custom.
“This is not a simple social convention, but rather the fundamental cell of every society. Consequently, policies which undermine the family threaten human dignity and the future of humanity itself,”
he said to some 180 diplomats at the Vatican yesterday. The education of children needs proper “settings” and “pride of place goes to the family, based on the marriage of a man and a woman.” I'm not quite sure that personal pride in being traditional and obedient to dogma constitutes justification for regulating private life in a free society, but then a free society is the definition of a heretical society and it wasn't long ago that Catholics were threatened with excommunication for voting for political leaders. I'm sure the rationalization for that was much the same farrago of gold embroidered, incense scented rubbish as was the persecution of Galileo and the scientists of the Renaissance -- to cite the less egregious examples.
The fact is, that children raised by gay couples can't be shown to have turned out as Benedict predicts and since gay people have been around since the dawn of humanity and a bit before -- and long before the shaman, that oldest of professions, told them they were evil -- we can assume that human dignity hasn't been much affected. The indignities of the Crusades and Inquisitions and centuries of war and tyranny might have done some harm, but I won't go into that here.
As I said, science rejects propositions, predictions and proclamations that do not produce the results claimed. I might suggest Mein Herr, that there's a bit of egg on your face and blood on your robes from trying to stifle that heresy with force of arms, torture and murder, but so far, nothing you or your predecessors have predicted has ever been demonstrated to be the truth. So how long caro padre, will you go on predicting that if we do A, then B will happen, because we've been doing A for a hell of a long time and there's no sign of you being right so far.