While not well known at the time the show aired, witchcraft (or Wicca) is a religion and not just "casting spells." While Harry could possibly face non-public political repercussions for having a relationship with a woman of a non-traditional religion, he would not be fired summarily. In fact, if it was proven that he had been fired because of the religion of his girlfriend, he would have an excellent lawsuit based on both discrimination and first amendment grounds.