It's funny how everything is multiple shades of 20/20 in retrospect. I can understand why adolescent males wouldn't really care about another guy sneaking over to his girlfriend's house to get laid, but I still don't understand how the father is blamed for 'spur of the moment' actions.
Did he wake up in the morning expecting to plan out a way of reacting to a situation? No. I've exploded over far less important situations. The father had no idea what was happening in his daughter's room. You guys watch the news just like everyone else. All kinds of messed up things are happening, so I can see the father thinking the worse. It may not excuse how he reacted, but why judge a guy for arguably reacting in a bad/violent way under these circumstances?
Personally, I kind of think that the entire situation is laughable. I don't have children yet, but when I do, I'll feel the same way a lot of other good parents will feel. Protective. Boys will still have the exact same thoughts I had when I was their age. From the father's perspective, maybe he reacted the way he did because this is a girl he raised and his parental instinct for her is that strong. Maybe he went off because he knew that having a naked boy in his daughter's room might lead to problems that most young teenage boys and girls don't think about. What if she gets pregnant? Is she in a position to care for a child? What if he gives her an std/vd? The kid might be messing with his daughter and several others up the street. Dads think like that.
Most of us have done things similar to this and it didn't seem like a big deal; maybe it wasn't. However, from an adult perspective, it could be, and that's probably why the father reacted that way.
You think this is bad? When I was in hs, I drove my cousin to a girl's house so he could spend some time with her over the July 4th holiday. I met her family and they were really sweet people, but it didn't prepare me for what would happen later. Basically, they wanted to screw each other, and I had no problem with it, so we decided to go to another one of our friend's houses.
Apparently, this girl didn't tell her father that she was leaving to go somewhere with 2 young males. Her mother didn't mind at all, but her Dad was pissed. When we came back to drop her off, we went inside to get some fireworks so we could go out in the backyard and set them off. Her dad comes downstairs into the living room. While we're in the kitchen, we see a camera flash go off. Then the father asks me to come into the living room with him. He shows me a picture that he's just snapped of him holding a 12 gauge, double-barrel rifle - up to the camera lens. He looks at me and asks if his daughter is worth that. I just met the girl several hours ago.
I wasn't fazed because a lot of my extended family carry similar firearms while they hunt, or in their trunks;when they start drinking, some say and threaten to do a lot worse. Anyways, I got the point. Was it my fault? No. I'd just met the family. Maybe it was because I was 17 and they were both 16. I don't know, but I could understand where the guy was coming from. In fact, he was a little bothered by the fact that I'd kept my composure when he threatened me. Later he brought out a bunch of bottle rockets, and we had a great time setting them off. He even offered to have me come back and work on one of his hot-rods. When we left, I never returned to that house. Was her father justified in assuming that I was up to something? No. I wasn't. It was my cousin. Did I understand? Yeah. Considering what had happened between my cousin and his daughter, I could understand where he was coming from. We could've been killed, but I could identify with her father.
All I'm saying is that it's a wacky world we live in; a man can't even forcefully control a situation in his own house? Next time he hears strange noises, maybe he'll call the police and wait for them to arrive. Maybe he'll just lay in bed while his house is robbed. Maybe he'll just ignore the fact that his underage daughter is letting boys in his house, possibly against his authority. Just so he doesn't
'react in a bad way' and get charged?

It kind of sounds like legal castration to me.