According the Pope Benedict (who occasionally makes sense), same-sex marriage is a threat to world peace because "Everything that serves to weaken the family based on the marriage of a man and woman, everything that directly or indirectly stands in the way of its openness to the responsible acceptance of new life ... constitutes an objective obstacle on the road to peace."
Here's the problem. There's no proof of any sort that same-sex marriage weakens one-man, one-woman marriage. None. I would argue that greater acceptance of same-sex unions will actually strengthen not only traditional marriage (by reducing the motivation for those who use marriage to reinforce their closet doors), but our society as a whole by reinforcing the stability that comes from couplehood.