I hear that, I definitely do. But what's done is done.
The first thing I would want confirmed if I'm OP is what exactly the friend said to the Wife. The friend might have assumed that the whole time OP knew they were talking about her, didn't say anything and then walked away. That does sound worse than Friend changing the subject and then OP walking away.
After that, here’s a full summary of all the points I would make to the wife if I’m you OP, /u/nohonor23.
(Btw, if any of this comes across as wrong let me know.)
#1 - I was not 100% sure they were talking about you, it was an assumption that was only confirmed when we got home.
#2 - If I confronted them and was wrong about it (ie. they weren’t talking about you), I look like a fool AND I’ve just revealed that I thought that my wife had done that (that’s not defending your honor).
#3 - Even if I had been 100% sure they were talking about you, picking a fight isn’t a good idea for at least 2 reasons:
a) People at the party would want to know what sparked the fight and word would have likely spread to everyone at the party. Now everyone knows.
b) Look, as much as no one likes to be talked about in such a manner, and I understand you are upset about that, if you take a step back the two guys hadn’t really done anything wrong to warrant a fight from me. It was two people talking about a sexual experience they had with a girl who did it willingly. No name was shared, they didn’t know you were married or that her husband was right there AND at the time, I wasn’t in a relationship with you, so it wasn’t like there was any cheating going on. So what exactly did I have on them? Nothing. Two men that happened to have sex with my wife before we were together, talking about it without revealing any names, that I only figured out was maybe you because my friend changed the subject. Honestly, once the subject was changed, wasn’t the best thing to do in that situation is walk away? So it doesn’t get out of hand? So it doesn’t get revealed to the other people at the party? There was no honor to defend. It happened. And if I was able to hear them talk about my wife willingly having a threesome, and not let it bother me because I know you have changed, why are you?
____________________________________________
Now, ugh, I hate to say this, but it almost feels like something else is going on here. Why is so strung up on him defending her for something that happened so long ago, before OP was in her life? (or in a relationship with her - I’m not so clear on the timeline about that.) Is she doing some wrong now, that she feels the need to be defended? I hope I’m wrong, but either way, OP’s wife’s sounds completely out of line here (and again, this is coming from a woman.)