Dude, that's rough.
My dad caught me once. He's real cool, but he was still pissed. He was a hippie back in the day, so he's like "I'm not going to stand here and tell you that I never had any of this stuff, but I regret it and if I had the chance I wouldn't do it again." He promised not to tell my mom if I never did it again. I think his only real problem with me smoking pot is that it's illegal, and he doesn't want me to get into trouble with the law.