About two years ago my longtime girlfriend and I finally bit the bullet and we "experimented" with opening up our relationship a bit. Basically I wanted to see her fuck another guy and she was up for it

We found a guy that she worked with who was moving to another city and she found him attracted so it worked out perfectly. I know a lot of people find that weird, but it works for us.
Since then we tried it a few more times with other guys that she was into. Again, it was awesome and fun and there were no regrets. But over the summer we tried again with this guy that we knew through mutual friends. He seemed cool, my girl found him attractive so we eventually invited him over for drinks and some late night fun.
Everything went along fine until they finally got down to business. He started to undress after my girl did and, I swear, he had one of the biggest cocks I've ever seen. I think I'm doing alright in that department, but he was just a monster. My girl started blowing him and, for the first time, I really felt uncomfortable. For the first time I felt jealous more than anything. It was still hot, but when it was all said and done, I was kind of hoping that she'd "complain" to me that he's too big or something...but she seemed to enjoy it just fine. For a little while after that, I had a bit of self-doubt whenever her and I got busy, but that eventually went away.
If she actually cheated on me and fucked somebody behind my back, it'd be nearly impossible to get beyond it and trust her again...but if I knew the guy was super well hung on top of it, I'm sure that'd sting even more.