Right after a long, stressful mission through a muddy, humid, awful jungle, while everyone is still sweaty and bloody and dirty, with the bodies of the targets still cooling on the ground, Bucky's handlers decide they need to destress before they get picked up by a team to take them home. It'll probably be at least a few hours before they can be extracted, and until then they have nothing else to do. They rape Bucky right there in the dirt, taking turns to fuck him. On his knees, with his fingers scrabbling for purchase in wet mud, on his back with their sweat and bloody scrapes dripping onto the fogged screen of his goggles and clogging his facemask, maybe even up against a tree, clinging to rough bark that scrapes against his bare skin while he's getting fucked raw from behind.

Just make it dirty and filthy and as horrible as possible for Bucky, who's just trying to grit his teeth and get through this surprise "extra mission requirement." Peeing on him is acceptable and mild gore is fine, but I'd prefer any fillers to avoid scat.