Leo:
Leo was looking for Percy and some of the campers had told him he was with Thalia in her cabin, so that's where he went. He knocked at the door, but no one answered and he made his way around the cabin to search for a window with open curtains. He wanted to check if someone was in the cabin or not. He found a window with a small gap between the curtains, but what he saw there wasn't quite what he expected.
He saw Thalia and Percy having sex - or more precisely - fucking, judging by the force the son of Poseidon used to ram into the girl from behind. Leo couldn't help but get a hard one as he watched his two friends. He knew he shouldn't do this, he should leave and act if he had never seen anything, but the temptation was to strong. He looked around, checking if someone were around but all the campers where in the training area and he couldn't see anybody near the cabins.
Leo slid his hand into his loose-fitting shorts, pulled his cock out and started to stroke his, in the meantime fully erected, dick. He grabbed his rod hard with his right hand and started to move it back and forth slowly. He groaned and started to fasten his pace bringing him more pleasure instantly. He looked back towards the two in the cabin and started to adjust his tempo to Percy's, making his movements even faster. He eyed Thalia, concentrating on her reaction, her closed eyes and obvious heavy breathes. He imagined how it would feel to thrust into her warm and wet cunt, aroused and needy for him to fuck her. He imagined how it would be to hear her loud moans and the noises his balls would make when smacking against her. He shook himself out of his thoughts and noticed, that he was near his climax and concentrated back on the pair in front of him. Percy pushed himself deep into the daughter of Zeus but didn't pull back, obviously reaching his climax and spreading his cum inside of Thalia. The black-haired girl on the other hand laid on the mattress with her ass held high, not moving. Percy had managed to fuck her senseless, Leo noticed impressed. As Percy still shot his load of cum into the unconscious girl Leo couldn't contain himself any longer and orgasmed too. He shot his cum straight into the small bush beneath the window. His now softening cock still in his hand he watched as Percy pulled himself out of Thalia's pussy and plugged his cum inside of her with a purple vibrator. As Percy finally made his way towards the door Leo quickly put his cock back inside of his shorts and walked of as if nothing happened.
But he couldn't get those pictures out of his head - he couldn't forget this sight, it played itself again and again and again in his head. Feeling himself hardening again, he started to walk in the direction of his cabin to get rid of the problem properly.