Well, I have had dad fantasies! When I was 18, me, my female friend, her mom and dad were enjoying their pool/ hot tub. Scott was wearing baggy swim trunks and when he got out of the tub, he had a full on erection. And it was BIG. He must be like 9 hard. I commented on it and his wife gave the "Oh Scott" routine. Also, the daughter was embarassed.
Later, when he was putting away things outside, I told him I was very impressed before. He gave me a stupid look and I reached out, grabbed his elastic band, pulled and took a peak. I smiled and told him it was as big as I thought.