A lot of guys request it. The step mother/step daughter fantasy does not involve fantasy of blood related incest. The mother/son one does.
Through talking to people who request these, (for fantasy roll play I require a little talk through at the time of the appointment to discuss boundaries, triggers, reason for the fantasy, and my needs to make sure it is safe emotionally and physically for both of us.)
Many have actually been sexually abused as a child by a parent, and over time it became fetishized. Whether the offender back in the day was the actual mother, aunt, etc still caused the desire for a mother/son role play.
How I've taken it, is that it creates a new memory that is safe and under the full control of the client. It's like looking back in time, and not clearly seeing the offense, because later in time a matching image, (that is closer to now,) is blocking the true image. (In emotion, sense of control, photo images, etc. etc.)
I believe it is the mind taking action/creating a desire to heal itself. It almost becomes obsessed with it for the person to pursue the fantasy until new images are burned into the psyche.
The new images sometimes being a hot younger girl of their own choosing taking the place of their mom.
But I so know that it is a real fantasy and a real lust. Some think about their actual family member. All in all, they have total control over the situation as an adult, and a lot of people are trying to work something out in their own head.
Sexual abuse causes a lot of confusion, and I would assume a hell of a lot of confusion as a child. The brain is very powerful, and stores memories forever and ever, (unless damaged,) which guides our every day actions, our planning, etc. Even if we don't consciously know the reasons for our consideration for current and future decisions.
So again, I believe the hard drive is trying to fix itself and measure, make sense of the large amount of past events that are using a significant amount of memory.
I don't think there is anything wrong with it, and in have actually pursued fantasies due to past sexual weirdness as an adult with uncomfortable situations. It has turned my sexuality around completely. It has also brought a lot of healing and understanding.