What's the complete picture as to what's happening here?

My guess, and it's only a guess, is that off camera to the right there's some sexual activity going on - like some guy getting fucked, but by someone on his knees? - and this line of guys (on the edge of a swimming pool?) are waiting their turn and getting their cocks ready to put them in to wherever they're to go. As I say, it's only a guess.