My friend that I asked on a date kissed another guy at a party the day before our first date. Why do I feel bad?

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I had been working with this girl for 2/3 months, we clicked really well, she liked me, I liked her, I found out she had a new job so I asked her out on her final day, on a Friday..
She said she would love to go on a date but had to go to her best friends birthday on Saturday first, so we arranged our date Sunday. We kissed on the date and a few months later we’ve just made it official. She mistakenly admitted to kissing a guy at the party when she was drunk. Why do I feel gross and wronged?
My friend that I asked on a date kissed another guy at a party the day before our first date. Why do I feel bad?
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