I googled the question because I was curious as to how many Catholics are leaving the Church over its stance on SSM (which in many quarters is becoming increasing socially unacceptable), and found this recent article. I wonder how over time the Church will handle this matter.
It's a bit dated, but Pew did a study on people raised Catholic a while back. Some relevant stats:
It seems like gay marriage is the primary reason for leaving Rome for a small segment of ex-Catholics, and part of a larger "package" of reasons for a much larger cohort. A couple other thoughts:
1) Catholicism is many losing people to Evangelicalism and Mainline Protestantism too, and for very different reasons from the "nones". They seem stuck between a rock and a hard place.
2) I was concerned that reasons for leaving would be influenced by nominal Catholics becoming nones. That is, people whose parents rarely brought them to mass, or taught them the faith leaving over homosexuality might give the impression that there is a crisis despite those who were raised in the faith holding firm. The study seems to indicate that there might be a bit of that here, but the effect is modest.