Why does my married coworker get jealous of me talking to other women?

I work in a office, and are friends with a married coworker. We are friendly text and joke but never to flirty just innocent fun. But I've noticed when I flirt with other women her demeanor changes and is seemingly jealous. She gets very irritated and starts acted distant and cold towards me. I doubt she is the cheating type so why would she be jealous?
Why does my married coworker get jealous of me talking to other women?
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