Well there are some extremists in any fan base but I've been to a couple of ND /Navy games and twice have sat in the middle of Navy folks. They were very good people and even better sports fans ... Even after I acknowledged that my service branch was Army. Lots of kidding, good natured vibes, and cheering for well executed plays regardless of the jersey colors.
In Jacksonville two years ago there was a father and son sitting in front of us. The father was USN, the son was ND grad. When I got back to my hotel in Atlantic Beach, a Navy Lt Commander in Blue Angels fatigues asked, "well, how'd we do?" I was wearing ND gear and replied, "You did well. We were aTD short." After he uttered some expletives, he explained he was an ND Man. Did premed at ND on Navy ROTC. Went to med school, entered the service, and when I met him he was the Flight Surgeon for the Blue Angels who had done an air show that day and were billeted at my hotel.
The relationship between ND and Navy still runs deep. Fierce competitors on the field, friendly foes in the stands.