I'm not sure if simple attendance is the problem so much as actively participating. For example, it is my understanding that you may attend weddings and funerals but not be an active participant in them. There is a balance between charity and duty to God here.My mom doesn’t understand that someone who is Catholic, technically, isn’t allowed to attend church anywhere else but a Catholic Church
I saw somewhere a large majority of the annulments granted by the Catholic Church are mixed marriages. There's a reason you need a special dispensation for this.