Regulus was meandering about, getting ready to have his bath, a towel slung about his waist. He'd had a mundane day, and he was expecting to have nothing but a very mundane shower before he settled down to do some mundane homework.
That's when he'd heard them. And the sounds they were making weren't the least bit mundane.
He couldn't help but blush at the knowledge that he could recognize his own brother's moan so easily. He followed the sound, peeking around a corner to see his own brother pushed against the wall as Remus Lupin ran his hands and lips all over him. Regulus couldn't be sure of exactly who was doing what to whom, his eyes instantly drawn to their twin erections, rubbing against each other as the water poured down on them, tiny rivers running over muscles and scars.
He felt his blush deepen and he retreated, cheeks burning scarlet. Couldn't they have gone someplace more private? Or at least any place where he wasn't likely to walk in on them. Regulus was a new Slytherin Prefect after all. Of course he'd be in there. Of course he might catch them. Of course he might see them.
Were they trying to rub something in his face with this? Gryffindor solidarity or something? Or maybe Sirius had seen the way Regulus sometimes cast looks their way, fidgeting and flushing, trying to think of anything except for the feel of someone's chapped lips closing around his--
Regulus gasped a bit when he felt his own cock start to harden. And in spite of the Blacks' penchant for inbreeding, he knew that it was beyond improper to get excited after watching your own brother have sex with another boy. No, that was wrong. That was very, very wrong. Never mind how exciting it was, how good it would be, how much he envied them, how much he wanted to take someone’s place although he wasn’t certain who.
He was starting scare himself, and he decided it was best to leave the showers and get away from the sound of moans and shower water pounding against bodies and everything else that would serve as a reminder of what he’d walked in on. He turned and prepared to go, running directly into a strong, flesh colored wall.
Regulus looked up in horror to see his brother grinning down at him roguishly. “Going somewhere little brother?”
Regulus was about to squeak out that he was going to go gouge his eyes out or something equally unlikely when a hand came down on his bare shoulder. He turned around a bit to see Remus giving him a smile that was oddly serene and obviously quite naughty.
And that's when he realized that they hadn't wanted it to be private, and they hadn't wanted to rub it in his face. He couldn't help but marvel at their cunning as Remus's hand slid the towel from his hips, felt Sirius's lip mash against his own, and lose track of who was who altogether.