You can scroll to the bottom of the walkthrough linked the main post for a better idea but generally all the main girls have four paths that are (not official names): Love, Friends, Corruption, and Domination. Nicki has a fifth path, Friends with Benefits.
As of this release, none of the paths affect any of the others (so you can be love with one girl, corruption with another, just friends with two, etc.) EXCEPT that you get hidden "darkness" points from domination and you need to have six of them (not that many, basically just means you need to have dominated one or two other girls) to see Pepper's domination scene. Don't do that, though. Be nice to Pepper, she's a cutie.
Also you can move between paths (for example, raping a girl on your first encounter but apologizing for it on the second will shift you from domination to corruption). The dev has said that eventually you'll have to make some choices if you're on a nice (love/friends) route in order to keep it semi-believable (no marrying girl and her mom and seven other girls all at the same time) but everything's still pretty early so it's silly to speculate on what may or may not show up a year or two from now.
