Opening her eyes; she looked over at Hermione and said; "I won't ask how you heard about "Pets" originally, but, it's a tradition that has almost exclusively been the province of the Pureblood families. Basically, the wealthier, and more powerful families have developed the habit of selecting particularly attractive, un-attached Half-Blood or Muggleborn witches, and keeping them as "pets" for lack of a better word. They are protected, and gain the status of being affiliated with a Pureblood in certain sections of society. In return, they are available for our pleasure. The reason I say, almost exclusively for the Pureblood is that I did some checking, as well as discussing it with my Aunt, and it turns out that certain people, because of their accomplishments, are able to break through this class barrier. In fact, it's almost expected that they would take at least one "Pet'. For example, Harry, you are "The Boy Who Lived" even though you hate that title, it's a fact. Because of that status, if you have aspirations of moving in Pureblood Society, which, from what Ron says, you do, then you need to adapt yourself to the idea of having a "pet" as a symbol of your status. You too, Hermione; not only are you with Harry, but you have achieved notice on your own based on your genius. Even though you're a Muggleborn, you will be expected to have a "pet" as well, if you expect to fit in with the older families."