Why do girls get so angry when their SO mentions being flirted with, but every girl talks about being flirted with all the time?

I've noticed in my relationships my entire life, girls constantly talk about guys flirting with them. Which is fine. But literally every time I've ever mentioned anything about a girl flirting with me, it's met with aggressive denial, or outright rage. No exceptions either.

Ladies, do you do this too, and could you give some clarity on what this is about?
Why do girls get so angry when their SO mentions being flirted with, but every girl talks about being flirted with all the time?
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