He shouldn't have stayed with Priya. Penny was and continued to be the love of his life. Even with the satisfying sex, Priya kept trying to change him. Change him by different clothes, wearing contacts and not being understanding of his love of playing video games. Don't get me started on his love of Science Fiction or Super heroes and comic books. Penny never tried to change him. She helped him buy suits for formal activities like university functions and dates. Penny loved his glasses and liked him the way he was. As Stuart once said, "Lenny together, makes one awesome person". Leonard and Penny belong together. Leonard and Priya, not so much. Plus, the first chance she got, she cheated on Leonard with her ex while in India.
Sorry, that boat has sailed already. Leonard wouldn't take her back even if he was lonely and desperate. Priya cheated on Leonard with her ex the first chance got back in India and she considered it no big deal. Now Leonard did kiss Alice and that is a small cheat. He did call and told Priya of the kiss and apologized for the kiss. By that time Priya already had sex with her ex and had no thoughts of telling her boyfriend. So, Leonard wouldn't take her back.