Should i stay with him? thats if he forgives me!!! :(?

my boy friend is SO angry at me and i completely understand why! he walk in while i was getting wiv one of his best friends! i dint know why i did it i thought i was happy with him but hes so .... safe, he never drinks, smokes gets in trouble!!! btw im 15 so its a little more serious for us lol. his friend is SO hot though and i think its some thing about a kind of (it sounds gimpy) bad guy ;) i have no idea what to do!!!! help! i know what i did was wrong so don'tt be harsh on me !