Makes sense to me. You had an agreement to participate in this intimate act together and she did it without you and without your consent. I don’t think that’s fair at all and I would be mad too.

Were you sent on a pointless errand?

Why didn't you call ahead and ask them to wait? Since the horse has left the barn, you ought to just bone the guy up the ass and send him on his way. Mission accomplished.

You agreed to a threesome. What she did was not a threesome. She cheated.

How TF is this a YDI? Threesome - sure, fine, let's roll. But if she slept with the dude without her partner, that's cheating.

So you didn’t seem ready for a threesome after all. Cause yes they should have waited for you, but not sure you would have appreciated it either if they had.

*Note to self don't be late for threesomes. Though seriously that situation really sucks.

Threesomes take way too many organizational skills

It’s not a threesome with just two people. You are the main dish, he is just a side dish. She was inappropriate for her action. She knew the terms, just as you did. She is grown. She should have patience. What she did was wrong. And honestly, this is your hall pass to have a “threesome” with another woman while she’s not around. But I’m sure she wouldn’t be too pleased with that. Make sure she knows it’s a threesome before it happens kind of situation. Or really just talk about it. Maybe if you go to couples counseling bring it up. They are good at their jobs and could help you come to a solve / resolution. What she did is cheating. No way around it. No way at all.

set up another threesome but with a woman and just do it with the woman and at the very least you'll be even....maybe your wife would even understand the discomfort of the situation