How do I respond to this?

So I have a good friend of mine who we have been close since elementary school but these days she is acting quite flirtatious and not acting like her usual self. She has been very sexual with me (telling me to undress her and to put one of her boob into her mouth) i have confronted her and asked her about this and she says it is nothing special. But she continues with this despite knowing i am taken. What should i do here?

How do I respond to this?
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