In that he is obsessed that only boys can like blue and only girls can like pink, only boys can like Thomas/Diego etc and only girls can like Fifi/Dora.
When we play with his train set, I am not allowed to touch the 'boy' engines and he will pile them all up next to him and give me the 3 'female' trains he has. He gets very stressed if I put a blue top on and he found an old pink top of his in his drawer this morning and was horrified when I told him it was his
There are loads more examples - is it normal to be so obsessive about the difference between girls and boys at this age?