First, I would not mention his name on your post, or I would make up a name just for the story. This is to keep you out of trouble.
About what happened... If I get it right, they were having sex in a common area, where you could go as you wish, right? If so, I don't see anything wrong on your part. You could have just walked by and he would have had nothing to say about it because you have as much right to be there as him. He decided to have sex there when he could be in a private place.
Now about the noise. It is also true that they could do it in a place where no one can hear them, but it's not always possible. So I can't blame them for that. Although, some people like it when they can be heard. If that's the case for them, then it makes it even less wrong that you peeped on them. They can't decide that they get to be heard because they like it but people don't get to watch them.
Again, I hope that I got it right and that they were not doing it in his private space. If they were, then I may not approve your behavior.
If it was in a common area, not only would I have peeped on them, but I would even have gone right in front of them and watched openly. And if he has something to say about it, I would ask what gives him the right to decide that no one can be in this lounge whenever he is having sex.