Why doesnt my boyfriend show PDA? help?! :(?

My boyfriend and I have a child together, and have been threw a lot. And he always says I love you and i know he means it but like in church and stuff he never puts his arm around me and even in public, and in school all my friends & his friends show affection! he only does it when were alone, when were together in public he treats me like his best friend but not GIRL friend :( why does he do that. i asked him why and he said " im not gay" but people who are straight do that stuff! and before we had our baby he was dating my sister and we got drunk one day and had our beautiful daughter anyways before that he always put his arm around her in public.. i feel like his best friend not his girl friend why is he doing this?