Why did she reject me then get jealous?

I liked this girl, despite being in a relationship with another woman (my current girlfriend), and she showed signs of liking me. One day, I made a joke and told her that “I thought she was my girl”. She responded that, “we were just friends”. We remained friendly with one another. Fast forward a month later, and she sees me and my girl out and about one weekend and she seems jealous.

When I see her the following Monday, after realizing that she must have had a change of heart after since she’s now jealous, I drop in a conversation that I view her as a little sister. Her face drops and she looks dejected.

Honestly, to show her what she missed out on, I kept, expressing to her just randomly in conversations that she is like a little sister to me and showing off my girlfriend (e. i.: boasting about what I bought my girlfriend for Valentine’s Day, asking her for advice on where I should take my girlfriend on a date, taking my girlfriend to places I know she’ll be at).

Meanwhile, she’s still single over a year later and having a tough time dating.
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Yes, I was already in a long term relationship, when I pursued her. I prefer to be involved with multiple women (as do many men).
Why did she reject me then get jealous?
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