The support of 18-29 year old Catholics for gay marriage isn't in line with the opinions of others in that age group of other religions.
Catholics are concentrated on highly urbanized states and their views aren't that different from those of their (Mainline) Protestant neighbors.
Before Vatican II it was very different. This is a case of Catholics finally assimilating to the opinions of the (secular and Mainline Protestant) culture after decades of being chastised for their "backwardness," and finding themselves attacked by people who really weren't on the scene for all those years.
On of these days, the Catholics will stop blaming their problems on non Catholic churches. If it isn’t the protestants, it is the orthodox. If not them, the free masons.
There was synod that just concluded where the Pope was saying nice things about gays, and it was blamed on the Protestants.
We all have issues, because we are all fallen. Stop complaining, start fixing