I think you misunderstand Roxy's character completely. Roxy is the type of girl who typically use sex as a way to avoid emotionnal connection with someone. Connect to her emotionnally and she'll stay loyal to the end. Now if her backstory is sexual in nature, it will only means that either she got emotionnally involved with an asshole who decided to share her, OR she lacked this emotionnal connection to begin with and get involved in something she later came to regret.
A slut knows what men likes, right, but a slut like Roxy will also learn what "her" man likes too. Take Ivy in comparison. Ivy is "a" slut. She doesn't use sex to avoid getting attached to a certain someone, she's just a slut who like having sex no matter with who. Roxy is way more complicated as a character. Once she's emotionnally attached to a dude, it's more likely that she stop being "a" slut and instead become "his" slut. Plus she'll be less easily influenced by assertive dudes or gals compared to virgins or innocent girls (Ashley as an example), because she'd have more experience both sexually and emotionnally. Now in term of "loyalty", I'd say that it balance things out.