Nostradamus: is it possible you are a little hyper-sensitive to the sexual predator issue because of having worked with it, and been exposed to the behaviors more than most people? The intellectual background you bring to such a situation (assuming lots of study about human sexuality) will be very different from other people.
Of course, good on you for leaving after your wife said she was uncomfortable. The moment she finds their horsing around for her benefit (if it was), and she felt creeped out by the situation, you took care of her and you and left. Excellent. I've felt frightened (in my younger years) by what I thought was inappropriate sexual behavior directed at me, and a more or less dignified retreat fits my personality better than a confrontation.
For you, though, perhaps it felt like you wanted to do more than just get her out of there and got temporarily stuck in the angry feelings arising from what felt invasive? I've been there, too, killing angry, but no reasonable way to express those feelings, and they do need to come out and be dispersed safely. Add to this our duty as healthy adults to protect and nurture children, and you have an interesting stew, so to speak.
I can imagine that this whole discussion may have been frustrating to you, partly because it drifted away from your specific situation to the general. It was the specific situation that torqued you, and those the feelings that needed venting. If I got it right about your ground of being, all I can say is that I am very much on your side, and for some of the same underlying reasons.
But still, generally speaking, I find nudity as an issue a non-existent issue; generally, it doesn't offend me. In this particular situation, your wife's distress
made it imperative that you take action, unfortunately the only reasonable action open to you was emotionally dissatisfying.
Sorry if anything I wrote here offended you. I have no wish to add insult to injury.