"I am unable to eat cow's milk" isn't entirely accurate for me, it comes down to a battle between my willpower and the knowledge the delicious butter-containing-cake I'm about to eat will most probably have me suffering a variety of allergy-like symptoms within seconds-to-hours afterwards.

Whereas I can't eat prawns because, well, I just can't bring myself to. Little black eyes, ick. The only time I've eaten prawns was when a friend made three-course dinner including prawn cocktail and I didn't want to disappoint her. :-) (prawn crackers are fine, yum, but prawn-shaped chocolates are a no-no too - how bizarre)