If they dont believe the tenants of the faith, they shouldnt marry in the Church. That doesnt mean they should attempt marriage somewhere else.
What you are asking, essentially, is for a person who doesn’t want to be Catholic to live a life of celibacy.
If someone who was raised Catholic rejects the Church, they are still going to lead “regular” lives in other aspects of life. This may include falling in love, getting married, and raising children. Since they don’t want to be Catholic, they’re not going to achieve this goal the Catholic way. They are going to achieve it another way—in another church, or strictly in the civil sense.
In their mind, they are married. In their mind, it’s the most important day of their life, especially for women. And in the eyes of the law, they are married. In another post, you yourself said you would refer to your hypothetical daughter’s invalid husband as your son-in-law in the civil sense. “Give to Caesar what is Caesar’s. Give to God what is God’s.” They wouldn’t be married in the eyes of God or the Church, but they would be in the eyes of the law. And they would be married according to their own beliefs and conscience. I’m not saying they
are married, since objectively speaking, only those married according to Church law are really married. But we’re talking about the perspective of a bride and groom who have rejected Catholicism.
So, to them, someone as close to them as a parent—especially a mother to a daughter—not being with them on what they perceive as their wedding day is going to be viewed as choosing a religion with strict rules—that the child doesn’t agree with—over their own child. That’s going to cause some very hurt feelings, especially when there
isn’t a concrete rule that the parent absolutely may not attend, and compromises
can be made (making it known that the parent doesn’t approve and hopes they will reconcile with the Church, etc.). I’m not sure it’s fair to compare this to a temper tantrum, in every case. It’s acting out of a hurt that runs deep. Because the parent can’t agree to disagree on matters of religion but still be there for their child on one of the most important occasions of the child’s life.
I hope you can understand that.