SUBREDDIT: r/relationships
TITLE: M [19] with my girlfriend F [19] made out with a guy the day we began dating, and never told me until 8 months later.
POST: My girlfriend who I've been dating for the past 8 months, which I've had a good stable relationship with, made out with another guy the day we began dating and waited until last night to tell me.  She seemed quite upset about it, although should I be mad that she lied to me about it for the last 8 months, or should I move on and forget about it.  I do love her, but because of this makes me think she might do something like this again and me already being a bit insecure makes this situation even worse.  I also don't have any tolerance for cheating.  

If she would have told me the day she did it, or a week or two after, I probably would have broken things off.  But now that we've been together for the last 8 months in a good relationship I don't want to break things off, I just lost a lot of trust in her for that.
TL;DR: 
Girlfriend F [19] made out with another guy the day we began dating, and told me 8 months later.  Should I be mad, or move on?