Female friend is not happy I like another girl?

Context

I'm friends with a gal I met in BioChem. I liked her at first, but she didn't reciprocate so I agreed to just be friends last year.

She recently heard I had feelings for a different girl in class, and reached out to "check in and confirm if this is true". I agreed, and told her all about the girl, but she did not take this well. Why would this be an issue if she was truly just a friend?

Female friend is not happy I like another girl?
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