Why do girls assume my gender if it's offensive?

Let's say I ask for their number at the gym after having introduced ourselves. I asked a hot girl for her number and she said, no she has a boyfriend. But wait, I thought assuming one's gender was offensive? I didn't say I was a guy or a male or whatever. I never assumed her gender either.

Politics aside, why does every woman (especially beautiful women) assume a man's gender just by looking at that person's facial and body structure?
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I identify as male, but let's say if I didn't identify with any gender and I asked a girl (or anybody) out and they said they have a "boyfriend". Would their intention be transphobic or genderphobic?
Why do girls assume my gender if it's offensive?
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