She has to respect your property. Let her know that the beer belongs to you. And let her know that your relationship has not evolved to the point of either of you taking the other's belongings without asking first. Tell her you are not comfortable with her drinking your beer and convince her that you are a germaphobe. Give subtle implications that you believe she may be infected with a contagious disease and you do not want to contract it via shared alcohol beverages. If this terms and conditions are not to her liking, she can go elsewhere as you are unwilling to compromise your strong beliefs and values for sexual favors.