“why would they excommunicated for just marching in parade supporting homosexuals?”
According to the article they were allowed to march as individuals but were specifically told not to march as a parish and not to represent that they speak for Catholics in general.
Priests take a vow of obediance. If they do march as a parish it is deliberate disobediance. And it is dishonest. They pretend they represent the Catholic Church.
I would definitely fire the priest who deliberately disobeyed. I would defund the parish.
“...allowed to march as individuals”? That sounds like weasel words.