I figured before my wife and I had kids that God would punish me and give me all girls, just so I would understand what I put several fathers through. Thank the stars I have two boys and only one girl, the boys are 7 and 3 and she is 5, right in the middle, maybe I wasn't so bad after all?
I have recently had to have a discussion with the two older ones about how daddy wishes he could be their buddy all the time but sometimes daddy has to be daddy. I tried to make it clear that even if they get in trouble and I have to bust them that I still love them, no matter what they did.
My wife is a labor and delivery nurse and I've suggested jokingly (maybe?) that someday she take them to work and show them just what it's all about. She works at the city hospital and unfortunately there is no shortage of underage kids showing up in the situation of the boy who could be king but clearly now is a dick and the girl being stuck.
I had been walked in on by both my parents and both situations came out very different. The situation with my dad started the same as yours seaweb, he was in the garage and we were downstairs, needless to say she left right after that. He called me out in the garage later and we had a talk, one of the few that I remember in my youth where my dad talked to me like an adult. While I'd like to say it changed me it didn't, I did begin using protection but still kept screwing around.
My mom, on the other hand, freaked out and called the girls mom, the girls mom came over directly and picked her up. Needless to say she banned her daughter from seeing me and I'm not sure what else but none of her friends would talk to me either after that. I did manage to get a moment with her at school sometime later and apologized, told her I didn't mean to get her in trouble and hoped she'd forgive me even though we couldn't be friends or date again. I found out years later from someone else that she thought what I said was very cool and didn't have any bad feelings towards me.
I think you handled this pretty well seaweb, even given the stupid stuff that has gone on since. I'm sure someday your daughter will see that daddy was just doing daddy's job and realize that you were right.