Cut one of the grapefruit in half, and reserve the other half. Cut the rest of the grapefruit (half plus one whole) into segments. Squeeze juice of the reserved grapefruit half into a small bowl. Add honey, salt, pepper, vinegar and oil to the grapefruit juice, whisking constantly until blended. Place lettuce in a serving platter. Arrage the grapefruit segments and avocado over the lettuce, and drizzle with the grapefruit dressing.