Because it is what gets taught in general in Catholic High School Marriage class and in pre-Cana. Also, most Catholic families have a gay sibling, child, parent or cousin who is gay who they would like to meet a nice and settle down with. When you relate the matter to your own family, suddenly Catholci gay marriage sounds essential. There is no doctrinal rebutal for the experience of the faithful. Or the gay priests who are most likely blessing civil gay unions privately. That last part is what has the hierarchy scared to death.