WHICH ONE YOU WILL PICK

Hey , im 17 years old and my parents been together for 22 years ( junior high school sweet hearts ) and married for 9 . I love both of my parents alot , but recently my farther have been cheating on my mother . I know this because when ever im with my farther alone ( mall , car , ect. ) hes on his phone with another women flirting and having small talk . Why he even feel comfortable enough to do this in front of me ? I dont know . The conversations with the other women are often sexual and I find it disgusting , however I still love my farther and have not said anything to my mom , but I feel like im betraying my mother . My mother do not have a clue on whats going on when shes not around . My parents rearly argues , and I hear them having sex almost every night . My mother is 34 and is a very attractive 5,6 rich caramel skin , long brown hair , beautiful curvy body , shes a hard working mother . She do not deserve this . They have been together since my mom was 12 , she has been with NO other man .What do I do ? Do I tell my mother ? Do I let her find out on her own ? I dont want to be the cause of a divorce *