Um, why is the Catholic church worried about any Mormon re-baptisms. They don’t actually believe that counts, do they?
It’s a matter of disrespect and arrogance.
Let’s say I created a ceremony to make all their ancestors gay. Now, the fact that they don’t believe in my ceremony doesn’t change the fact that my doing so is disrespectful to their grandparents and the choices he/she made while alive. It’s arrogant of me to assume that anyone but myself wants it and that I have any sort of power to do so.
It doesn’t matter if I can smile and say...”But your grandpa can still choose not to accept homosexuality up in heaven. This just gives him the opportunity to be gay since he didn’t know about it in this life.”
It wouldn’t be a homo vs. hetero argument at all...it’s a simple case of respect for someone’s completed life.
In a short few words: respect for the dead.
It is an insult to people who may have died for their faith to say that their faith may not have been enough after all.
Members of the LDS have, at various times, baptized Jews who died in the Holocaust, Catholic martyrs, and so on.