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There is no reason to play dumb with her as everyone knows what had transpired. Since this is your home where this had occurred, you can talk to her about respecting your space in the future by not bringing male companions when you're not there. You may think about taking her key for the time being.
As far as being sexually active at 20, it is not too young. Your main concern may be that she uses protection and does not get pregnant before she graduates and is on her own two feet.
She is an adult and you can talk to her as one and let her know that your expectations of her are reasonable (what they should be of you interacting with another adult)