Daddy!?

My boyfriend always wants me to call him daddy, usually before we're getting busy, I bust out laughing and it makes me think about my dad. After that I'm done and can't even think about doing the deed. He also likes to sneak up behind me and grab my ass and say whose your daddy all sexy like. I can't help but say my dad's name!! At this point it's more of a game. I wanna get over it but I can't!!!! Advice please :)