While I am the first to admit that I don't follow the news as closely as I should (especially this time of year where I am quite swamped with work) here's something that I think is more amazing than the fact that this was approved (or rather in this case it was already approved. The President simply decided not to veto it): no one is talking about it.
Seriously, apart from a few minor news blurbs this whole thing has gone rather quietly. Sure there have been the traditional Church complaints, demands that this be taken to referendum, and even a wacko protest march (I remember a chick about my age saying that if gay marriage was approved, Humanity would go extinct. I don't think I have ever felt a greater sense of embarrassment while watching the news) but there hasn't been a fraction of the debate and controversy than say the American media.
If I were to hazard a guess as to why, I'd say it's because everyone here is more worried about the new tax increases as part of the economy recovery plan or whether our economy is going to go the Greek route than dudes marrying each other.
Anyway, I'm glad that this went through nevertheless and hope other countries follow the example!