Made a Joke During Sex and Triggered My BF, Now He Doesn't want To Have Sex Anymore...

I have a dark sense of humor and made a joke at the wrong time. I heard a story of a guy having sex with a girl and he would say crazy stuff while he did it, don't remember all the details but I thought the story was hilarious and wanted to try it my self. One line from the story really stuck with me and decided that is what I'll say. While doing it doggy style he starts moaning and I take this as my opportunity.Me - "You like this dick?"Him - "Fuck ya"Me - "You like this dick you fucking retard?"After I said that he looked back at me I have a huge grin on my face and I'm trying all I can not to start laughing. He says hes done and gets dressed didn't say much and just left. After texting him later that night he told me that he was called retarded a lot in elementary school and it always bothered him. I didn't know this and I apologized like crazy. Now anytime I try to start sex he is standoffish. Any advise on how to mend are relationship?PS Im 18 hes 20.TLDR: Called my BF a "Fucking retard" during sex as a joke and he freaked out because he was called retarded in elementary school.