Most Helpful Guy
First, you give him a few days to maybe a week to clam down and in that time, leave him be. He's your dad--he'll forgive you. When it's time, apologize for not locking the door and tell him that you didn't tell him about you and your boyfriend because you knew he wouldn't understand. That you'd like to put it behind you and move on with your life together.
Most Helpful Girl
You don't have to do or say anything. You should probably give him some space. He needs to realize that his girl is growing up and taking a shower with a boyfriend is normal by our societal standards. I am sure he will eventually get over it.