My boyfriend cheated on me?

Me and my boyfriend went and picked up my friend from work as her car was broken down! I get out to let her in the middle as she's smaller then me. After about 10minutes of her being in the car , she starts touching my boobs, at first i thought she was joking around until she ripped my shirt off and then my boyfriend whipped his penis out!!!!! I freaked out. And yet she still tried getting on top of me! I told her to stop and for my boyfriend to knock it off . They both looked at me and started laughing. Saying "babe" come on it'll be fun. As i try to get the truck keys, she leans over and starts sucking on my boyfriends penis. After that i was so freaking frustrated that i pulled up my skirt, and jumped on my boyfriend quickly and started fucking him while doing so ,i was pulling her hair to piss her off but she seemed to love it! The fact here is , did my boyfriend do this on purpose? Or did he have something going with her ?? Should i leave him ? As I'm writing this hes laying on my lap. We got home and i ran to the bathroom cried and got over it for the sake of our kid's! :(