There are good reasons to talk to your roommate about this. First it is annoying to you and you have a contractual right to a descent night sleep. Second it is a different person each night, suggesting that it is a virtual stranger to both of you. That is not safe from a security point of view. As a person you may be concerned for her well-being, even if she is not.
Promiscuous sex is stupid.
Speak to her about your concerns. Be clear that you are unwilling to relent and that you will be honest with the hall director if you have to ask for another room. Almost all roommate problems can be resolved with communication, but not all communication solves problems. It has to start with the person who is bothered, and has to be reasonable.