Thank you all for your replies.
The distinction I would draw between a mistake and an accident is that you *make* a mistake whereas an accident *happens* to you. I don't think this happened by accident, which is why I straightaway wanted to understand what it all meant. She thinks she was perimenopausal and in mild midlife crisis and it won't happen again.
There were two separate incidents that night. One was with another woman, and one with a man. The former went as far as two women can go, but the latter went no further than that, as far as I now know.
Both are friends of ours. Both are married to other friends of ours. One of the things that frustrates me most is that she doesn't really face up to how she has wronged the other halves. Mind you, she is not the only person either OW or OM has cheated with. (She knew this about OW, but not about OM at the time.)
My overriding immediate response was of relief that she had told me.
Having said that, she could hardly have avoided telling me about OW. I had been making cocktails for our guests in the kitchen but later came into another room and found her and OW kissing very passionately and publicly. I froze and left the room. I went to the loo to gather myself. I returned and they were gone. With a sense of dread I went up to our bedroom and found them in there, already half undressed.
I should mention that 20 years ago, this had happened before with the same woman. I had put it down to the youthful experimentation she would have had if we had not got together so early in our lives (she had never had any sexual partners before me). I should also mention that this woman had also made passes at me, and I had been sorely tempted, so I sympathised somewhat.
But back to me standing at the door of our bedroom. My wife came and kissed me but told me she was "just feeling into [OW] right now". Still in shock, I left the room. I did not go back. I avoided her for the rest of the night. She went to bed before I did. When the last of the guests had left, I went to bed in the spare room.
I did not sleep. After about half an hour, she came to the spare room. She had clearly not slept either. She was very ashamed. She told me what she had done with OW, and then told me about OM. With him she had kissed and they had put their hands in each other's clothing (I did not ask for details). I was glad she had been truthful about it. She was desperately amorous, and I really needed to feel close to her, so we made love.
For the next few days I was just trying to make sense of everything. I told her how much I valued her honestly and this prompted her to admit that things had actually gone further than she had admitted with OM. He had gone down on her. This is when it started to feel wrong for her, and she had shut it down with him.
It also transpired that things had gone further with a colleague of hers than I had previously known. The weekend before the party she had told me that she had kissed this colleague (let's call him OM2, even though he was actually prior) on a couple of occasions when they were drunk and flirting had gone too far. I had taken this somewhat in my stride because she was basically telling me it was over now (although admittedly this seems like it was more because of the wife's suspicions than her own regret or his). In the aftermath of the party, when I was trying to impress on her how much I wanted her never to keep something from me just because she was ashamed by it, she took the opportunity to tell me that she and OM2 had also been pretty handsy with each other.
I know how all of this sounds. You must think I'm kidding myself. But I believe that this is the extent of it. And if that's true, it doesn't even come close to threatening our marriage. I'm a pragmatist. I don't have any great moral attachment to monogamy. This was not a prolonged affair. It was a mistake. Not an accident, but something to be learned from.
As I mentioned, the overall effect of this has been to make me fall in love with her all over again. To start with, I think she needed this hysterical bonding too. But I was needy with it, which we've never really experienced in our relationship before. We've always been fiercely independent as well as extremely close -- which works when you want each other but don't need each other. But my limerence started to give her the ick. I understood this and toned it down, but in truth I've just been masking it.
To some extent, like her I just want things to go back to the way they were. But for me they cannot. I cannot compartmentalise it like she can. I sometimes wonder if I'm idolising her as a defence mechanism. Whether this will all come crashing down at some point. We've always been able to talk about anything, but we can't talk about this, which is maybe what hurts the most. Also the things that have made our relationship work are more difficult for me now. I've always felt that a person should be able to have their private life -- for example that she should be free to flirt if she doesn't rub it in my face. But I feel that was maybe predicated on being able to talk to her about anything. But what if she comes to resent me not wanting to be so independent now? I have genuinely valued that aspect of our marriage over the decades. But something has changed now, and I don't know how to deal with it. It's been seven months and it doesn't feel like it's getting much easier.