I still don't understand how people get in situations like this. If my daughter-in-law kissed me randomly out of the blue the only person I'd be sitting down to talk to is her at that point, I'm basically her father. The girl was emotional and obviously acting rash in a way that could never work and I would need her to acknowledge that. From there I would consider whether to let the event go as a fluke or talk to my son about it.

It has no implications for me, my marriage, or my wife. In this situation I'm 46 and married and this is my son's wife. The idea that I would ever allow this go any further is preposterous. I wouldn't hesitate to tell my wife about it if I wasn't convinced it was a fluke, but I just can't imagine being with a woman that would freak out and try to turn me into some kind of villain in this situation. And I can't imagine acknowledging or engaging with that kind of drama for even a second. There's just no way my wife could hide that kind of crazy irrationality for decades or however long we've been married in this hypothetical situation. We would've split up long ago or never married and I would be unmarried or have a wife that doesn't act like an idiot.

Basically, you've married a drama queen that suspects you're a cheater, you know this about her at this point. Maybe she has good reason to suspect you from your past actions and you've failed to mention this. But either way you knew this would be her reaction. It doesn't matter though because you still need to do the first thing, talk to the girl and figure out if this is a problem for your son's marriage. It's not a problem for you, it never was. If your wife decides to make something out of nothing that's her business, never engage with that kind of behavior.