Agree with the others... sounds like he just got caught up in the moment with the bum out, music going, dancing around, having a bit to drink, acting silly. They all likely had a bit to drink and got caught up in the moment when he let his bum out.
I can't imagine she would mean anything by it with her husband there, in public, and knowing a camera was on... that is likely part of the reason they did it... for the camera, just not thinking.
I understand your not liking the situation, but I do think it was a one off slip up. Hopefully he will learn something from the experience... especially the part of it upsetting you!