<human>: Could you distill the essence of the text into 1-3 sentences?

So I will try to keep this short but happy to give more detail if anyone has any questions.

My girlfriend (1.5 years) has traditionally, had a lot of guy friends and was FWBs with three of these (before we met). For the first six months I tried to be okay with this, for a large part, because I had a number of close female friends (although none with a sexual past). We've largely gotten over this now (since moved to a new city, living together, more mutual friends).

However around 6 months into our relationship, I had a particular issue with one guy friend (let's call him Pete). A couple of weeks after we got together, Pete had made a move on her (she told me about this at the time).  He also constantly offered to give her dancing lessons and used to take her to clubs (I'm not much of a dancer). My GF was always insistent that nothing untoward happened on these nights out and that Pete was always just a friend.

As I said this was all about a year ago and Pete has since moved city and we have put it behind us. However, based on a few things mutual friends have said and things she said in a fight last week, it emerged that there was more to these nights out than I had thought. 

Basically, she has admitted that he made a move on her again on one of these Salsa nights, sent her inappropriate pictures, asked her to visit him in his new city etc. It now seems that there had been a lot more emails / texts than I had originally been led to believe.

Anyway, basically, I feel that I was mislead and betrayed. My GF agrees and has apologized but also thinks, because it was a year ago, its not such a big deal. We are in a much better place now and are a lot better at communicating with eachother.

So the real question I want to get advice on is, am I wrong to be so angry about something that happened over a year ago? Would it be better to just forget about it, especially since they're not still in contact?
<bot>: GF was hit on by a guy friend one year ago and didn't tell me at the time. Am I right to be really angry or should I just let it go?