See, that's my point. Your mom and sister were obligated to each other. The aunt broke the sister bond. Shrillary and Monica had no bond, they were nothing to each other. Bubba, on the other hand, did had a bond with his wife and the nation. Of course Monica and your aunt are not innocent but that's apples and oranges. As for the Commandment, it was that a man may not covet his neighbor's wife. Rather sexist but there's nothing there about a single woman going after a married man. And if I remember correctly from Catechism class, it is the married person who commits adultery but not the single person because the married person has the commitment of marriage whereas the single person doesn't.
ANY sex outside of marriage is a sin . IMHO