For a young man in his prime, the most familiar and unguarded female body (pussy) is his mother. On the other hand, a mother with a mature body that she can't fully satisfy knows it's wrong but can't resist the irresistible pleasure... Because if it's not tabooed as a social norm, it's inevitable that mother and child will indulge in pleasure together, so it's deliberately made into a taboo, such is the theory that crosses one's mind.
