If you're only interested in 2D, I might suggest Kingdom of Deception. The MC isn't "technically" a slut from the beginning, but it is more because of her pride than because she isn't into sex. As time goes on though she's a bit more depraved. Regardless, you'll probably enjoy it. You might also like Good Girl Gone Bad. Again, not exactly a slut from the start, but it itsn't difficult to get her to start doing slutty things. Only problem is its pretty much a pure VN.
If you'll take a chance with 3D I suggest Coceter Chronicles and Chloe18.
Coceter Chronicles is my favorite adult game hands down, its got a little something for everyone, but most importantly, great characters, setting, story, and world building. Its main character, Tabitha, is exactly who you want her to be. Early in the game you're asked to find the cook's son, after he was sent on an errand he should have been back from by then. You find him, his back to you, in a dark corner of the cellar, "grunting". You're given an option to approach him, or stand back and watch. If you approach him, you catch him just as he's about to cum, startling him, and having him turn and cum all over you. If you stand back, he finishes before you ask if he's all right. In his embarrassment, either way, he runs away. You, discover, he was masturbating to a picture of an Alcurian, from the book of races (alcurians are a race of futanari angel things but if you're not into futa content that's fine, you can disable it) and you're given a choice to touch his cum. If you touch it you're given a choice to taste it. If you taste it you're given a choice to decide if you like it or not. And so on and so forth. The entire game is like this, giving you full choice over how Tabitha reacts to any given situation. It does, however, have stats, and if you make her too submissive, or don't train her fitness enough, or don't make enough slutty decisions there are some options that will be unavailable to you.
Chloe18 follows a girl desperate to become a cheerleader. In the first major scene of the game she loses her virginity on the beach to some random guy she just met. From then on its a game of exploiting just how slutty you can get her to behave. You have to train her fitness, sluttiness, charm, and intelligence, and later exhibitionism, but once you do she's down for just about anything. This game is an over the top fuck fest with farcical writing and whatnot, but I love it all the same.
