The subject of Taboo or Incest is often treated as forbidden content in decent circles. Yet most stories are not a single label to be rejected out of hand from some self-righteous bias or pre-conceived notion. Some scenarios defy being put in a box.
Young Andrea is "over the hill" at 21 which has become the cutting off year for eligible young females to gather in a spouse in normal circumstances. The Doctor has arrived on the scene to save her from impending spinsterhood and she is grateful beyond belief.