I'm nervous to be writing this, as if doing so is acceptance of everything I've essentially been in denial about for a while now.
A bit of background about me, I'm 25, my girlfriend is 24, we've been together for 4 years now.
Until a few months ago, cuckold fantasies were not something I had ever considered I might get. I've known about cuckolding since my early teens when I stumbled upon cuckold porn. It's not been something I was all that into, I found something about the way the women are in those videos hot but it wasn't something I wanted for myself.
My best friend since school has always liked to joke about fucking other guys' girlfriends/sisters/mums while they watch, I'm not entirely sure why, it's mildly amusing sometimes but everyone just accepts it's something he likes to joke about. He'll often call guys cucks (which isn't uncommon with most people I know) and he'll joke about how they'll be in awe as they witness his "prowess", stroking their dicks in arousal. He'll do all kinds of actions and impressions, I thought he'd have grown out of it by now

I was never the target of these jokes until I got with my current girlfriend. As he says, she's out of my league and I know it. He loves to tell me what he'd love to do with her, how I'd watch in awe, how grateful she'd be etc. I never found this arousing or considered that I could get aroused by it before. I just took it as a joke, a compliment to me and her and kinda laughed it off. He also likes to jokingly flirt with her in front of me and she will jokingly flirt back, both of them trying to provoke a response from me but I just roll my eyes and try not to encourage it.
About 6 months ago he broke up with his long term girlfriend and has been on Tinder since. Around Christmas time he was showing me photos of some of the girls he had been with or had been chatting to. I was flicking through the photos on his phone (he wasn't watching) and came across a dick pic. I quickly flicked passed it but for some reason went back to it. I guess it didn't seem real and I felt the need to check why. It was definitely him but his dick was huge, not just long but girthy, veiny and with a big head on it. I paused for a moment, assuming it was fake but couldn't see anything fake about it. I carried on flicking through photos and found another 2 dick pics, same huge dick from different angles. He had always bragged about having a massive dick but I just assumed it was a joke, I never had any reason to believe he wasn't average. I carried on looking at the girl photos, handed the phone back and didn't give it another thought.
The next time my girlfriend and I were having sex, I looked down at my humble dick between her pussy lips and this thought entered my head that I couldn't shake. I wondered how much his dick would stretch her lips, how erotic it would look to see that big cock inside her, how much more she might enjoy it. I was shocked to be thinking about it, I tried to think about something else but kept coming back to it, imagining her on her knees sucking on it etc. The thoughts made my dick harder and more sensitive, it felt good and I came hard a few seconds later. She looked at me all confused, asked if I was ok, I tried to shrug it off but felt immense shame for thinking about it and for enjoying the thought. I tried to convince myself it was a one-off, a freak thing that wouldn't happen again but I avoided sex for a few days, I think because I was afraid what it might mean if those thoughts reoccured.
A few days later the same thing happened, the session ended early because the thought entered my head and made me cum again. I was scared that I couldn't resist or control my thoughts. Unless I was horny, it was the worst thing imaginable. As soon as I was horny, it was impossible not to think about. I avoided masturbation for a few weeks but everytime we had sex the thoughts were there waiting for me, becoming more vivid and extreme. I wanted something to prove to myself that I wasn't a cuckold. I hesitantly searched for some cuckold porn, something where the actors looked similar to us. I convinced myself that I wouldn't be turned on by it, wouldn't want it to be us. That day I saw cuckold porn in a new light, it hit hard and fast. I resisted stroking, knowing how shameful it would be to stroke myself to the thought of my best friend ploughing my girlfriend. I was constantly trying to rationalise that I didn't want it but I did. I couldn't stop watching, couldn't stop imagining, before I knew it my dick was in my hand and I was shooting ropes of cum. Before I had barely finished cumming, I realised what I had just done, the porn was still playing, the thoughts I was enjoying just seconds ago felt like a punch to the gut, like the worst imaginable situation. I closed the porn and tried to busy myself with whatever I could to take my mind off it.
A few hours later I felt like I needed to watch some more cuckold porn to "prove that I didn't want it" again. This time I picked something with a different looking bull, I guess thinking it would be easier or at least not make me think of it being him. The same thing happened again and everytime since. Sex too has followed the same pattern, me trying to focus on her but inevitably my mind gets hijacked and things are cut short by me getting over excited.
One thing was clear at least, that I in no way wanted this to happen for real, as long as I wasn't horny at least. In my head I was sure that if they were to do anything more than joke flirting I would put a stop to it. This was tested a few weeks ago when, I can't remember why but she bent over near him with her back to him. He stood up, said something jokingly provocative to her, she turned to smile at me as he got behind her, pulled her ass into his crotch by her shoulder and slapped her ass. She let out a little whimper and they both giggled, looking at me to gauge my reaction. This wasn't overly unusual but has gone up a level since he's been single. The difference is I wanted to convince myself I could stop it but I couldn't. I found myself frozen and curious. Luckily they stopped there and laughed it off but I've replayed it in my head over and over and I feel like I wouldn't be able to protest. Once the horny fog sets in I feel like I would continue to watch and almost will them on. It pains me to admit it but that feels like the reality and that scares me.
I've been searching this forum for posts from guys that have had a similar situation but they are few and far between. Most of them posted their situation but never posted an update to what happened. In general, it seems like most guys here have always been into it and are wanting to experience it but does my situation ring true for anyone else? If so, what happened next? I don't want to be into this but, if I'm honest, I can't see myself being able to give it up either, it turns me on like nothing else and the orgasms are next level. I think my main worry is that sex will never be "normal" again. I can't last as long and can't stop myself thinking about him with her. Also I worry that the joke flirting might expose me somehow.
I feel like no good can come of telling my girlfriend about this. She would have too much fun teasing me. Every time he jokingly flirts with her, she'd know what I was thinking and probably encourage it to turn me on. I also feel like given the opportunity, she would fuck him. Telling her would probably start a chain reaction to it happening and I can't bare the thought of that, it would ruin me.
All and any guidance would be greatly appreciated. My heart is pounding from writing this.
Thanks for taking the time to read.