A phrase referring to a woman whose hotness is something more than just raw sexual desirability. What makes her sexy is not measured in base terms like "how much would you pay to bang her," but rather by whether or not she is someone you would want to engage in a long-term or permanent relationship with. Usually, a girl who is "girlfriend sexy" possesses at least some wholesome, nurturing qualities and a good sense of humor in addition to being physically attractive. She's the kind of girl who you can bring home to meet your parents, and she usually has good mom potential.
Keep in mind that this definition is fairly subjective, since what makes a girl "girlfriend sexy" will always be dependent largely on what a guy is looking for in a girlfriend. Generally though, they are considered to be more "authentic" than the average hot chick, which can mean a lot of different things to a lot of different people.
Guy 1: "Why are you so hung up about that chick from your art class? There are plenty of chicks in this school who are sexier than her."
Guy 2: "You just don't get it-- she's the only one who's girlfriend sexy."