I was recently at a dance music festival and me and my friends were having a great time and rolling pretty hard. My girlfriend(27f) wasn't able to go and stayed home. My 2 best friends (27m) and (27f) are in a relationship. While we were vibing and just generally dancing and hugging each other and what not my male friend told me to touch his hair because he does a really good job with it so it's super soft (even more so cause of rolling). After touching his hair he kissed me on the forehead. Completely fine, I'm not worried about that just a friendly gesture. But then his girlfriend (also my best friend) said, "NO! not like that. On the lips!" So without thinking we just went in for a really quick kiss. It's not romantic in any way. I do find him attractive and I am bi but he is 100 percent just my friend and I've never thought of him that way. Anyway I immediately felt bad about it. Fast forward to the 2nd day and we were rolling even harder than the day before and while we were talking about random stuff it came up that I had never actually kissed a guy before and after I told him that he pulled me in for another quick kiss. Again, from his end or my end it didn't feel romantic or even sexual. I proceeded to feel very bad about it though and kicked myself for not even thinking about it and going for it both times.
I told them I felt bad and we obviously decided I should be fully honest and tell my gf what happened. They all seemed to think she probably wouldnt care at all. However I'm very concerned. My gf is very monogamous and from 3 years of being together and knowing everything about her I'm genuinely concerned that she is going to be very hurt and consider it cheating.
I'm really just looking for advice on how to approach the subject and how to talk to her about it. I'm also curious if anyone else has had a similar experience.
TL;DR
I (27m) kissed my best friend (27m) at a dance festival. What do I say to my gf(27f)?
Thanks!