With all due respect to His Holiness, I don't see how same-sex marriage threatens opposite-sex marriage. I live in a state where same-sex marriage has been legal for a while, and know some opposite-sex married folks here, and the passage of that legislation has not visibly negatively effected a single one of their marriages.
I am surprised that this Pope, who is normally so logical and compassionate, would lay something like this out there without being more clear about the logic behind it. Of course, it's notable that he doesn't actually mention same sex marriage by name in the article. His spokesman was asked about it and said that it was an example of what the Pope was talking about. I often get asked for a direct quote from the Pope and not a media summary or quote from someone explaining what the Pope says when I mention his progressive economic and environmental stances. So, in that light, I think it's fair to mention that he didn't directly say the words same-sex marriage or a synonym for those words here, though I don't doubt what his position on this subject is.