You're supposed to guage a womans modesty before making her your girlfriend. The worst thing you could ever to is try to control a modern woman's outfit selection. She will view you as insecure and lose respect for you. If she dresses as a slut, you should only use her for casual sex. Remember that women are the gatekeepers of sex, men are the gatekeepers of relationships. After months of hooking up, if she brings up a relationship via the "So.... What are we?" conversation, that's when you calmly state that women who dresses as she does are good friend with benefits material but not girlfriend material. You have all the leverage BEFORE the relationship, but if it is during the relationship she may see you as controlling.