I go away youth hostelling a few times a year with a mixed group of friends. I've shared both a 4 bed room with one couple and a 6 bed room with mixed sex singles (some of whom have partners/spouses elsewhere).
We have no issues getting changed in front of each other - I've seen everyone in their underwear (even the married men). I'm not really bothered who sees me in my bra but wouldn't be topless and change bottoms discreetly.
I'm going away with some different friends in a few weeks time. I'll be sharing a room with a couple who are married. We've already seen each other in various states of undress because we've been camping in a group together. So I shan't be taking myself off to the bathroom to get dressed and I wouldn't expect either of them to either.
I do find the comments about it being 'disrespectful' to the men's wives/girlfriends a bit odd. Along with suggestions that he should have told you. I can see how it might come up in a "And then I turned round and X had got their kit off!" conversation but not in a "I must inform you of something" way. I don't think it's disrespectful to get changed in front of someone else if you have a partner. My body is not owned by someone else.
I find that the 'social norms' around getting changed/undressed etc aren't quite the same in group situations/hostelling/camping as they are in every day life. I wouldn't dream of getting changed in my doorway for the neighbours to see at home but think nothing of getting changed with my tent open for the extra room/light/air even when there are strangers milling about.
However, I'm trying to imagine a scenario when I'd be totally naked getting changed and I can't really. That would feel unnecessary. But just because I can't imagine it, doesn't mean other people wouldn't.
So, to sum up, I don't think the girl getting changed is much of an issue. I think taking and keeping the video is. I think the lying is. And I think deliberately showing you is.
To be clear, I'm a 45 year old woman. I'd notice if there were a naked woman in the background of a video I was taking - esp if the background was just the other side of the room was in. A 20something bloke? He definitely noticed.
I think he was well aware that you would see it when he showed you the video. And I think he showed you to make you feel exactly as you are feeling now.