My partner was cheating on me with my closest friend

I walked in on my girlfriend and my best friend hooking up. I was devastated, but also turned on. I couldn't help but watch as my friend took my girl. He was rough, but she loved every second of it. I could see the pleasure in her eyes as he pounded her. I was angry, but also aroused. I couldn't help but stroke my own dick as I watched. I knew I had to do something, but I also knew that I couldn't resist. I joined in, fucking her from behind as he fucked her from the front. It was a sight to behold, my girlfriend being taken by my best friend. We went on for hours, all three of us lost in our own pleasure. It was the best sex I've ever had, and I knew that I would never be able to forget it.