After Mario told Katarina to give you a blowjob, go to him again the next day and Mario will force Katarina to fuck. Then he'll say something indicating that he actually has something else planned, which turns out he wants to divorce Katarina and fuck his daughter, Emma, instead but he will need your help to corrupt Emma first.
Go to the Sauna to meet Emma, do this 3 times ( once a week ) and tell her that she is beautiful and perfect until she becomes horny and wants to have sex. Carefully pick the dialogue indicating Mario Sharing Route and tell Mario that she is ready. Go to the Sauna again the next week and Mario will join in the fun.
So the real objective in Mario Questline is corrupting Emma instead of Katarina. But corrupting Katarina is also necessary to get her new scene at the Mall.