What is marriage as we define it?
Depends on who you are and where you get married.
For Catholics, marriage is a sacrament which takes place in the Church.
For non Catholics, it can be done at the JP’s office and is a simple paper indicating a legal contract between the two. IE civil union.
The Church does not recognize this as a sacramental marriage if Catholics choose to get married in this fashion.
So if a man and a woman get married, 1, 2 or 5 times outside the Church, the Church shouldn’t and doesn’t concern themselves with what is done outside the Church. The same holds true for if a man and a man or a woman and a woman decide to do that same thing.
That doesn’t mean that the Church approves of folks getting married 1, 2 or 5 times, or takes a position that those people aren’t committing sins. If the person’s aren’t members of the Church, and aren’t presenting themselves for other sacraments, it isn’t of concern for the Church.
No Church teaching has changed regarding approval of same sex civil unions based on what the Pope has said. Just like no Church teaching have changed regarding different sex marriages and divorces outside the Church.
Why is this so hard for folks to grasp?