So I'll use different names for obvious reasons. GF (23, we'll call her Emilia) and I (25) are both in the same graduate school program. We've been experimenting with cuckolding for the past 6 months or so, mostly with guys from online dating sites and once with a guy she met at a bar. She usually plays alone because I don't like the awkwardness of meeting the other guy. It's been a ton of fun for both of us.
We always had a rule that anyone in our social circle was off-limits, but that's made challenging by the fact that my roommate (also in our program, we'll call him Brian) is an absolute Adonis: tall, dark, handsome, athletic, he legitimately looks like an Abercrombie model. He comes from money and just carries himself with a quiet confidence (that turns into brash cockiness when he drinks) that women are drawn to.
Being friends with him has been a new experience for me, in that I'm used to being the guy that girls tend to go for. With Brian around, it's completely changed. Girls faun over him when we go out. We have several married couples in our circle, and he and a couple of the married women always toe the line between friendly and flirtatious when alcohol is involved. He handles women in a way I've never seen, putting his hands on them in public in a way that's not at all objectionable and socially acceptable, though if any of us other guys tried to touch these women in the way he does, it would not be at all acceptable. I could go on, but suffice to say, this guy is a stud and I think is the source of my cuckold fantasies.
So obviously, Emilia and I have fantasized about and roleplayed her and him having sex over and over again, and the sex is always insanely hot. She seemed to accept the fact that he was off-limits and was content to sow her oats online, but I could not stop cumming to the thought of them having sex, and while stroking off to the fantasy for the hundreth time I just texted her, "We have to make you and Brian happen." She was shocked and replied, "lol what? I won't argue but I thought he was completely off-limits". She obviously didn't argue very much and was incredibly excited to make this happen.
We both wanted things to happen organically, so we decided she would start to reciprocate his flirtatious behavior and just see where it went, and it worked like a charm. Watching them flirt, steal glances at each other, get handsy in a way that I couldn't have objected to even if I wanted to (because each act by itself could be dismissed as friendliness) was agonizingly sexy. They even started getting lunch and coffee together every now and then, and Emilia said they were very flirtatious when I wasn't around (she said once, "His face was really close to mine and I wanted to kiss him but I was just shaking and couldn't move, it was intense.").
Long story short, they had sex and she said it was incredibly hot and "passionate, like a sudden release of built-up tension". He also turned out to be incredibly well hung ("You were the biggest I'd been with before him, but....he makes you feel really small").
The second hottest part of being cucked by your friend (after the chemistry that your friend and your girlfriend naturally already have) is the way in which relationship dynamics change afterwards. Emilia and I are having crazy hot sex that nearly always involves Brian-related roleplay or details of their sex sessions. My dynamic with Brian has changed quite a bit too. He's much more assertive and acts as the "leader" when we go out places.
He tells the hostess how many people are in our party, he initiates conversations with waitresses and Uber drivers, he's just become far more socially dominant. He also throws me little jabs when I try to talk assertively to him, particularly when we drink ("Hey, how did I taste this morning?" or texting me a video of Emilia swallowing his cum). Emilia also has become the primary target of his drunken flirtations when out with friends, including one time recently when we were having chicken fights in our apartment's pool, and Brian scooped Emilia up on his shoulders. Everyone looked around uneasily, and the funniest part was the married girls he usually flirts with were both clearly aggravated by being passed over, and they had to hide their frustration behind the fakest smiles you've ever seen.
Anyways, being cucked by a mutual friend is much more real, hot and intense than by a random guy. I do fear that their chemistry will negatively impact my relationship with Emilia, but neither of us intend this to be a serious, long-lasting relationship anyways, so we're both just having (a lot of) fun with it.
I could see why a married couple might not want to do this with a friend, but for those who are in a position to do this with a friend, I very highly recommend it.
Me and Emilia