My fiancé cheated on me

My fiancé went to Krackow for my brothers stag party last weekend. Today he confessed that he kissed someone when he was there. He said it was a massive mistake and that he was so drunk she kissed him and he kissed back for a few minutes and then they walked away no numbers exchanged etc. I never thought I would hear those words from his mouth. I trusted him 100% but now that's destroyed. I'm numb. I would never do that to him. Should I leave him or give him another chance?