You had “agreed” – but it was clear that she was intending to have a relationship with him whether you agreed to it or not – and she had also made it completely clear that their “exploring” of their feelings for each other would involve sex.
[cap created a few years ago by https://www.tumblr.com/bbc4wife-cuckold ]
And then, even after they finished that go round, your wife called in to you to laughingly "dare" you to go sleep in the guest room so the two of them could sleep together in the main bedroom. And it was even later, as you lay there alone in the bed in the guest room, that you realized they had planned for the game to end up like that all along -- and that this was definitely not the first time they'd had sex.
This is based on a true event. I told my wife about my cuckold fantasies very early in our marriage. We would sometimes talk about them during foreplay, but they were never more than fantasies at that stage. One night my wife went out for dinner and drinks with some of her girlfriends. The next day she acted all serious and told me a made-up story about making out with a guy. I totally fell for her story and she teased me about how excited I got.
The moans and grunts stopped. You peeked in to see if your wife was OK. She was in heaven.