Pansy was probably just "convenient" for Draco while they were in school. In DH, Pansy was the one to suggest giving up Harry to LV which would have made her "bad news" to the Malfoy family trying to rehabilitate themselves and their image. I think that Asteria Greengrass was from a pure-blood family that did not support LV (at least openly). All of the Slytherins did not side with LV. Cases in point, Slughorn, Phineas, and Snape. I would have to assume that not all of the people sorted in Slytherin were LV supporters though they may have been "pure blood bigots". The Malfoy family would have sought a marriage for Draco other than from among the death eater families.