My mom saw my boyfriend's hand squeezing my ?

My mom called me saying that she would be home from work late so I decided to call my bf to come over to my house and we watched a movie in my room and ordered pizza. I totally forgot my mom could come home any minute so I started to make out with my boyfriend and he had his hand in my pants. A few seconds later, my mom opened the door and looked really really angry. I was mortified and I was speechless...to make matters worse, she hates that Im dating a guy 2yrs older than me(I'm 14)....what do I tell my mom? We were never going to have or anything related, HOW DO I EXPLAIN?!?!?