For your girlfriend's birthday, she asked to mess around with another man... but in exchange you'll get to mess around with her best friend. You're all in the same room, but you can't look. You have to keep your eyes on her friend. She teases you, telling you how long she's wanted you and stripping slowly. She encourages you to stroke while she touches herself, all the while you can hear your girlfriend's moans. She even describes what's happening just a few feet away. Then she starts stroking you while touching herself, making eye contact until you cum... in perfect timing. (Originally a custom. POV style, you see the "friend". Full nudity and masturbation visible, handjob is implied out of the frame. There is a voice over for the kissing/sucking/moaning of your "girlfriend". Some details of your girlfriend with the other guy are given, but there are no themes of humiliation. This is a swinging fantasy, not technically cheating since it's with permission, but it was the best fitting tag)