Sorry, but I don't think being Catholic or Christian has anything to do with ostracism by a mother. It sounds like she is mentally-ill. Maybe she needs help? Maybe she is beyond help?
Actually, it's a value judgment. We apply our expectations of motherhood or religion or culture based on our set of values and expect others to share the logic or priorities. They don't.
In her case, I don't find her behavior bizarre in the least, only sad and hurtful. People do stuff like this every day, all day long, in every society in the world. The whole bit of disowning and shunning was one of the key themes in
Fiddler on the Roof, and helped make it appealing to folks the world over. It could have just as easily been set in an Amish town, a New York art gallery, a military base, or a gang in LA.
Societies shun members for all sorts of specific things, but in essence for only one: breaking the taboos of the society.
Zombie's mother is living within the values of her society, and her real society isn't the multicultural inclusion of the larger American ethos, but the more immediate spheres of her religion and probably her ethnic heritage on top of that. Neither is known for being accepting of gays.
Within her mind, her world is right and its values are just, even though we see them as intolerant or hateful. Her actions are her ultimate acceptance of them, and her enforcement of them against what she sees as possibly a disobedient and willful son are just evolved continuations of how she saw her role in parenting when he was a boy. Of course, she only sees him as a boy. He will never be an adult in her eyes.
Zombie, is any of this true? Do you think it explains in part her perspective? I wouldn't presume to assume in specific like this, but I am postulating based on what I have seen in other first-person examples.