Nudity, liberty, curiosity. Combine those three and you'll have a large range of possibilities to play and explore with others with that same curiosity. Compare, touch, talk. I'm not talking about obsessing with sex and thinking about copying some porn scene you saw somewhere. I'm talking about exploring, mainly. Being playful with the bodies of your many mates present. And allowing them to be playful with you. Being able to smile or laugh at that shared curiosity and seeing it unwrap. Break the social conventions: feel free. For someone curious I think these kind of games are mindblowing.
But I guess it's difficult to find the right people with whom to share this experience. The guys in this set apparently found each other and it looks like they had a lot of fun. I think it isn't even that much sexual, more than it is an exploration of each other. (not sure if I expressed that last sentence right)
I suppose it can be considered a bit naughty, but why renounce to exploring these feelings which don't come very often?