Pascal, a physician in Plassans for 30 years, has spent his life cataloging and chronicling the lives of his family based on his theories of heredity. Pascal believes that everyone's physical and mental health and development can be classified based on the interplay between innateness (reproduction of characteristics based in difference) and heredity (reproduction based in similarity). Using his own family as a case study, Pascal classifies the 30 descendants of his grandmother Adelaïde Fouque (Tante Dide) based on this model.
Pascal has developed a serum he hopes will cure hereditary and nervous diseases (including consumption) and improve if not prolong life. His niece Clotilde sees Pascal's work as denying the omnipotence of God and as a prideful attempt to comprehend the unknowable. She encourages him to destroy his work, but he refuses. (Like other members of the family, Pascal is somewhat obsessive in the pursuit of his passion.) Pascal's explains his goal as a scientist as laying the groundwork for happiness and peace by seeking and uncovering the truth, which he believes lies in the science of heredity. After he shows her the Rougon-Macquart family tree and demonstrates his refusal to sugarcoat the family's acts, Clotilde begins to agree with him. Her love for him solidifies her faith in his theories and his lifelong work.
Clotilde and Pascal eventually begin a romance, much to the chagrin of his mother Félicité. (She is less concerned about the incestuous nature of the relationship than by the fact that the two are living together out of wedlock.) Félicité wants to keep the family secrets buried at any cost, including several family skeletons living nearby: her alcoholic brother-in-law Antoine Macquart and her centenarian mother-in-law Tante Dide. When Clotilde's brother Maxime asks Clotilde to come to Paris, Félicité sees this as an opportunity to control Pascal and access his papers to destroy them.
Pascal suffers a series of heart attacks, and Clotilde is not able to return from Paris before he dies. Félicité immediately burns all of Pascal's scholarly work and the documents she considers incriminating. The novel, and the entire 20-novel series, concludes with the birth of Pascal and Clotilde's son and the hope placed on him for the future of the family.

Question: Why does his mother disapprove of Pascal's relationship with Clotilde?
Answer: They live together before getting married
Alternative answer: Pascal and Clotilde are living together while not married