As you grow up, you are going to stop looking at your parents as superheroes that are perfect. I'm sure I'm not alone thinking my parents were superhuman and always had right answers for everything. Slowly, you are going to start seeing faults. I believe every one of these adultery situations is a case by case basis, where very rarely it is justified, but only you knowing the all the details can properly make this judgement if your dad acted outta cowardly behaviour, or was put in a situation where this was the best solution.
As someone who was in your situation at your age 8-9 years ago, I love my mom and understand her decision. My dad worked all week as a truck driver, and when he came home he got drunk. However, I still see my mom's decision as weak and agree with you that cheating is often the low road when both partners are capable of fulfilling each other, but choose not to. Only in certain cases, I believe, like physical handicap or overwhelming mental issues where they cannot fulfill, even if they wanted to, is there some arguable morally ok reason to stray a bit