If you said the same thing to him, I don't blame him. Nothing in your post suggests that you've grown in the slightest, you're just giving excuses rather than admitting to what you did. If on the other hand, you did own up to what you did then maybe there's a chance for you.

Cheating doesn’t automatically mean sex. Making out with another boy IS cheating. Although at 15, people make stupid mistakes. Talk to him.

Oh WOW this raises all sorts of red flags, for 1 your ex contacting your boyfriend was incredibly petty and childish of the ex and 2 your boyfriend believing anything your ex had to say, well that rings volumes about his own insecurities; also him holding it against you for something you did when you were a young teenager!! I would consider that a red flag as well !! 🤷‍♂️

you still cheated. once a cheater always a cheater. YDI

If you told him the fact that you were only 15 at the time, and he still has second thoughts, then you are honestly better off without him. Young people make mistakes, so that the older you has learned the lesson. If he cannot understand that, then I bet he will be insecure about you forever. FYL

All the comments are so bitter... relationships don't even count in highschool. My high school gf 'cheated' on me like this and I barely cared then and I could not imagine someone else caring now. OP must feel like everyone else has gone crazy