I watched my girlfriend Amy being fucked by two guys, tied down, while I was hidden in the closet. Nathan's friend was fucking her pussy, with her legs tied apart and back, her head off the edge of the bed, while Nathan had his cock in her mouth, at the same time cruelly pulling her nipples and slapping her tits. I could tell from Amy's sounds that she was in pain, turned on, getting off, and scared at the same time. Meanwhile, I was hard as a rock, my cock sticking out my fly, a bottle of Jack Daniels in my other hand. I had the door open only a tiny crack, and knew they'd never notice me unless they walked right up to the door and looked through the crack. The wildness of what was going on, along with the whiskey, finally got to me enough that I wound up just pulling down my pants and boxers. There was my girlfriend, being roughly fucked by two guys, one being the guy she'd been cheating on me with, and the other someone she'd presumably never even met before.
As I was marveling at the sight, I was absentmindedly putting the cap back on the bottle, and as if in slow motion, just as I tightened it, the bottle slipped from my fingers and crashed to the floor. It didn't break, but the bang was loud. The guys stopped, obviously, and looked over. Nathan strode over, his cock bouncing, and flung open the door. There I was, pants around my ankles, hard myself, caught hiding while watching my girlfriend be used. He kind of smirked and said to Amy, "Know this guy?" She was already looking over at me and slowly said, "Oh... my... god...". Both guys now started to laugh, while Amy looked horrified.
Suddenly something in Nathan's friend seemed to trigger, and he jumped off the bed, his cock still having been in Amy, and practically ran over, grabbed me by the arm, and pulled me out. He violently pushed me into an armchair facing the bed and told me not to move. He went to the dresser drawer that we keep supplies in, and grabbed some lengths of bondage rope. He tied my legs to the chair legs, and wrists to the armrests. My pants and boxers were still around my ankles. Amy was looking at me wide-eyed upside down, her head off the edge of the bed.
Nathan said thank you, and got back in position, between Amy and I, to resume fucking her mouth, and his friend got back on the bed and started slapping her pussy pretty fucking hard. Amy's cried were muffled by Nathan's cock, but I could tell that it was pretty hard for her. Shortly, the guy resumed fucking her. I was again hard as a rock, and aware that they were going to do whatever they were going to do, and I just now have a better view. I had a vague thought in the back of my mind that it might be tough later to explain to Amy what I was doing watching from the closet, but that thought quickly faded.At some point, Nathan came over and whispered in my ear, "Are we cool?" I kind of grinned like a jackass and nodded, and he loosened my ropes, but left them in place. Amy now was in the doorway to the hallway, her hands tied up to the chin up bar there, facing away from the room. Nathan, clearly very serious, mouthed to me, "STAY!". So I was to pretend I was still tied up? Fine, I thought, this is too much fun.
He went over to Amy and his friend, his friend currently standing in front of Amy, his cock in her pussy. Nathan started slapping her ass, alternating with fingering it, and eventually slid his cock into her, so she was being DP'd while standing up.
Eventually they took her down and tied her spreadeagled, face down, on the bed, her head at the head of the bed, her pussy facing me. Nathan took a belt of mine that was on the floor, and started whipping Amy's ass, hard. Amy cried out with each blow, but of course couldn't do anything about it. Nathan's friend now was standing to the side, stroking his cock, watching the abuse. I could see Amy's ass getting red, but I didn't move, being way to into the whole scene. Nathan angled some of the blows to contact her pussy, and Amy literally rose off the bed when that happened, clearly in pain.
The friend took a butt plug from our drawer and when Nathan stopped with the belt, slowly worked it into Amy's ass. He untied her and tied her again to the pull up bar, but facing into the room this time. I was waiting for whatever was going to happen next, very hard, but didn't expect what they did. Nathan walked over to my chair and spun it around, showing how I was not even tied anymore, then pushing me out of it. I couldn't catch myself, and fell on the floor. I looked at Amy, and her expression was as shocked as I'd ever seen it.
Nathan and his friend got dressed and sandwiched Amy, stroking her ass, pussy, and tits, both kissing her deeply. They said, "We'll be here at 9 tomorrow. Be showered and answer the door naked. We'll have clothes for you- we're going out." Then looking at me, "You'll be staying home". With that, they went out the door and were gone.
Amy still looked shocked, and I was still on the ground, my pants still around my ankles, and still hard. My mind was swirling, and I couldn't believe what had just happened. I was looking at my girlfriend, tied naked to a pull up bar, having just been thoroughly fucked by two guys while I watched. A lightswitch flipped in my head, and I got up and pulled off my clothes, leaving myself naked. I grabbed to belt from the floor and walked over to Amy. Her eyes got even wider, if that was possible, knowing what was coming.