He and his first wife were divorced after making the Passion. She was an Anglican, he implied in interviews that she was going to hell, which couldn't have been great for their marriage. Mel had a problem with alcohol when he was with her as we all know.
He ended up with that Oksana woman who was a Catholic. She turned out to be a gold-digging whore and backstabber. She goaded him into yelling at her then recorded it and tried to blackmail him for millions with it.
Now he's with a 24-year old named Rosalind Ross.
Obviously according to Catholic doctrine divorce and remarriage or sex is adultery. Who knows how Mel justifies it to himself. He's a sede so in theory he should be even more strict but maybe he justifies it by saying the church would annul those marriages if it was legitimate, or maybe his private church that he funds gave him the go-ahead. Or maybe he's really just losing his religion. Or maybe he knows it's wrong and just does it anyway.