The "Straight" Guy

Recently I have become friends with this new boy in my class. We joke around most days and I like to flirt with him every now and then just to see the response I get which is usually laughter. After a while I've begun to notice now whenever we talk he asserts the point "I'm straight" at least 10 times throughout class to me. Can someone please tell me why he thinks he needs to assert his sexuality to me so often???