Having lived about half of my life in Delaware, I wanted to chime in here so everyone doesn't get the wrong impression. Though I have returned to live in Pennsylvania, I still work and shop in DE and have never had a problem with my kilts there. But I have only been kilted in northern DE.
Delaware is very unique, surprising for such a little state. The northern part is very liberal and mirrors much of the NE US views, socially. The southern part, below the canal, or as it is sometimes referred to as "slower Delaware", has a very southern attitude.
This holds true until you get to the beaches. If you check out the license plates at the beach, you will see that probably 50% are from out of state, so I wouldn't hold DE responsible for the attitudes you received at the beach.
Bethany Beach is very money oriented, and many out of staters own places there.
Dewey Beach is the party beach. Very youth oriented, best bars to hit when your down there.
Rehoboth Beach is the place to be for 'alternative lifestyle' people. Very open minded. I just got back from spending a week there, and had no problems whatsoever.

I'm not sure where you camped while there, but if you were mingling with the locals, I would expect you to receive a very southern attitude towards the kilt.

Scott