21% of Catholics in Northern Ireland support unionism.
Potentially voting to maintain the status quo on 'just want to grill' grounds is not the same as 'supporting Unionism'.
Well, the status quo is unionism, so if the unionists decided to attack nationalists for being close to the Catholic Church like Jolly Slugg suggested, I'm sure that many soft unionist Catholics would reconsider their positions.
Historically the UUP at least did have a few Catholic members, but its fair to say they were generally regarded (by both sides) as somewhat eccentric.
The Alliance is to a large extent unionist, yes? I consider the Alliance to be a unionist party even if there are some members who support reunification.