His family hates me now......πŸ˜ΆπŸ˜πŸ€πŸ”«

Please tell me I have the right to slap him cause his family thinks it's wrong tf. Okay so me and my bf were chillin at grandma's house watching TV and his cousin comes thru. His cousin is 16. Well usually when I see my man's family I give them all a hug for a hi and bye. So my bf gets up first tells him wussup and my bf went to the bathroom right after. A few minutes later I got up and gave him a hug hi and this little 16 yr old boy (his cousin) grabs my ass but he did it quickly so I thought it was by accident or something. Ive known his cousin since he was 12 or 13 i didnt have any thought like that. So I let it go but I did give him that look like it better not happen again. After all that about an hour later me and my bf were getting ready to leave. So my bfs cousin and his grandma were in the living together and we went to approach them to hug and say bye I told grandma bye first then his cousin and he did it AGAIN! felt his whole hand on my ass with a little quick squeeze thinking he was slick so this time I know it wasn't no accident so i slapped him as soon as he took his hand off my ass. He's lucky I didn't knee him in the balls I felt so disrespected especially since he knows his cousin is dating me. His grandma turned so red and told me to leave. My bf didnt say a word idk why.. I respect her so much I love her to death it broke my heart when she told me to leave. She didn't see what happened (idk how) so all she saw was me slapping him. I couldn't even explain myself to her for the reason why I did it. My bf took me home but I saw so much anger in his eyes in his driving. From walking out thr house and into his car and on the way home he didnt say nothing to me. I didn't say a thing to him either... i was confused of the sistuation.. cause as if this point idk who saw what he did. All they seen was what I did. He's 16 a junior in high school he knows better it's not like I hit a 7 year old boy. So I felt like I had the right to slap him. I strongly felt like if I didn't he would probably do it again and again and again. Who knows. It's my body I never gave him permission to slap grabb touch my ass in any shape or form. I'm 21 I am not gunna let a high school boy take advantage of me like that nor harassment me