A mother is the most familiar and defenseless female body (pussy) for a young man in the prime of his life. On the other hand, her mother, who is too busy with her ripe body, succumbs to the uncontrollable pleasure even though she knows it's wrong. A theory that comes to mind is that if it is not made a social taboo, it is inevitable that mother and child will devour each other's pleasure, so it is deliberately made taboo.
