My girlfriend had a threesome

I knew she was thinking about doing stuff with the girl and I didn't want her to without me there. I don't like it anyway, but being there would make it a little better. We got into a fight and she went ahead and did it anyway. She told me that the guy got involved and she wasn't expecting it and that he touched her and she didn't like it, but she felt pressured and didn't want to say no. She just wanted to experiment and see if she was into girls. She said that the guy wanted her to go down on him and she felt too guilty so she said no. This is the first time she's done something like this and she says she won't do it again. Should I forgive her? Is this cheating? I told her I didn't want to talk to her for a week and she's really upset. We're high school sweethearts and she says that it would have to happen sometime growing up where she'd have to experiment with girls.