Wearing a bra at work

I am a waitress part time at a local coffeehouse/restaurant. I rarely ever wear a bra because they're uncomfortable and I don't think I need one (but do I even have to justify my decision??)
Don't get me wrong, I don't wear ridiculously revealing or see-through shirts, I certainly don't flaunt the fact that I don't wear a bra. But my manager approached me the other day and told me I need to "cover up" (wear a bra) otherwise there will be concequences.
Is this even legal? Can she do this? Can I be fired for choosing to not wear a bra?

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